TJA1041/1041A high speed CAN transceiver

AN00094
TJA1041/1041A high speed CAN transceiver
Rev. 03 — 8 November 2006
Application note
Document information
Info
Content
Keywords
Controller Area Network (CAN), ISO11898, Transceiver, Physical Layer,
TJA1040, TJA1041, TJA1050, PCA82C250/C251
Abstract
The TJA1041/TJA1041A is an advanced high speed CAN transceiver for
use in automotive and general industrial applications. CAN (Controller
Area Network) has become the de-facto standard protocol for serial
in-vehicle bus communication, particularly for Powertrain and Body
Multiplexing.
The TJA1041/TJA1041A features the low power management known from
the Fault Tolerant CAN Transceiver TJA1054. Accordingly, the
TJA1041/TJA1041A is predestined for Electronic Control Units (ECUs),
which are continuously supplied by battery regardless of ignition key.
Furthermore, the TJA1041/TJA1041A offers enhanced diagnosis features.
Local failures, like short circuits between pins, as well as bus wiring
failures are detected and reported to the host microcontroller.
Interoperability with the high speed CAN transceivers PCA82C250/251,
TJA1050 and TJA1040 from NXP Semiconductors is also considered.
AN00094
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TJA1041/1041A high speed CAN transceiver
Revision history
Rev
Date
Description
03.00
20061108
Updated version
20040603
•
The format of this application note has been redesigned to comply with the new identity
guidelines of NXP Semiconductors.
•
•
•
Legal texts have been adapted to the new company name where appropriate.
01.00
20020925
20010419
Update of Section 6.3 “ESD protection”.
Updated version
•
•
•
•
•
•
•
•
•
•
02.00
Update of Section 6.2 “Application of common mode choke”.
Update of chapter “Target Application for the TJA1041”.
Update of chapter “Hardware Application of the TJA1041”.
Update of chapter “Undervoltage Detection”.
Update of sub-chapter “Vcc Supply Buffering”.
New sub-chapter “Application of common mode choke”.
New sub-chapter “ESD Protection”.
New sub-chapter “PCB layout rules”.
New chapter “Wakeup Detection”.
New chapter “Pin FMEA”.
New sub-chapter “Differences between TJA1041 and TJA1041A”.
Updated version
•
•
•
•
New chapter “Target Applications for the TJA1041”.
•
•
New chapter “Under-voltage Detection”.
Update of chapter “Hardware Application of the TJA1041”.
New chapter “Flag Signalling”.
Previous chapter “Diagnosis” split into chapter “Bus Failure Diagnosis” and “Local
Failure Diagnosis”.
Update of chapter “Interoperability”.
Initial version
Contact information
For additional information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: [email protected]
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TJA1041/1041A high speed CAN transceiver
1. Introduction
The TJA1041/TJA1041A high speed CAN transceiver Ref. 1 from NXP Semiconductors
provides the physical link between the protocol controller and the physical transmission
medium according to ISO11898 Ref. 2 and SAE J2284 Ref. 3. It has been developed to
address mainly those control applications within automotive electronics, which remain
supplied by the battery during the whole lifetime of the vehicle. Focusing on these
applications the TJA1041/TJA1041A offers a low power management similar to that of the
Fault-tolerant CAN Transceiver TJA1054 Ref. 4. According to this concept one or more
external voltage regulators within the Electronic Control Unit (ECU) are controlled
autonomously by the transceiver. This concept allows a TJA1041/TJA1041A entering
Sleep mode to switch these voltage regulators off, disabling the VCC supply of the
transceiver and the host microcontroller.
The TJA1041/TJA1041A is available without packaging (bare die) as well as in an SO14
package as shown in Figure 1. The upper part of the SO14 pinning is compatible with the
SO8 pinning of other high speed CAN transceivers from NXP Semiconductors, like the
PCA82C250 Ref. 5, PCA82C251 Ref. 6, TJA1050 Ref. 7 and TJA1040 Ref. 8.
TXD
1
GND
2
Vcc
3
RXD
14
STBN
13
CANH
12
CANL
4
11
SPLIT
VI/O
5
10
VBAT
EN
6
9
WAKE
INH
7
8
ERRN
SO8-type
CAN-Xeiver
Fig 1. Pinning of the TJA1041/TJA1041A
2. General application of high speed CAN
A general application of high speed CAN is illustrated in Figure 2. Here a linear bus
topology is shown with the ECUs connected to the bus via stubs. Both bus ends are
terminated with 120 Ω, resulting in the nominal 60 Ω bus load according to ISO11898
Ref. 2. Figure 2 shows the Split Termination concept, which helps improve the EMC of
high speed CAN bus systems Ref. 9. The former single 120 Ω termination resistor is split
into two resistors of half value with the center tap connected to ground via the capacitor
Cspl.
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BAT
Voltage
Regulator
Vcc
CANH
CANL
CANXeiver
Vcc
INH
μC
+
CAN
RxD
Sensor
Actuator
TxD
GND
ECU
ECU
ECU
ECU
RT/2
RT/2
RT/2
RT/2
C spl
Cspl
Fig 2. Application of high speed CAN
The block diagram in Figure 2 describes the basic structure of an ECU. Typically an ECU
(CAN-node) consists of a standalone transceiver and a host microcontroller with
integrated CAN-controller, which are supplied by a voltage regulator. While the high speed
CAN transceiver needs a +5 V supply, new microcontroller products are increasingly using
lower supply voltages. In this case a dedicated voltage regulator is necessary for the
microcontroller. The protocol controller is connected to the transceiver via a serial data
output line (TXD) and a serial data input line (RXD). The transceiver is attached to the bus
lines via its two bus terminals CANH and CANL, which provide differential receive and
transmit capability. In the case of the TJA1041 there is an additional INH signal line
(indicated in Figure 2) controlling the voltage regulator. Leaving control over the voltage
regulator(s) for VCC and μC supply voltage to the TJA1041(A) allows for an extremely low
ECU quiescent current.
The CAN controller outputs a serial transmit data stream to the TXD input of the
transceiver. An internal pull-up function within the TJA1041 sets the TXD input to logic
HIGH i.e. the bus output driver is passive in open circuit condition. In the recessive state
the CANH and CANL pins are biased to a voltage level of VCC/2. If a logic LOW-level is
applied to TXD, this activates the bus output stage, generating a dominant state on the
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TJA1041/1041A high speed CAN transceiver
bus line. The output driver CANH provides a source output from VCC and the output driver
CANL a sink output towards GND. This is illustrated in Figure 4 showing the block
diagram of the TJA1041(A).
The bus stays in recessive state if no bus node transmits a dominant bit. If one or multiple
bus nodes transmit a dominant bit, then the bus lines enter the dominant state overriding
the recessive state (wired-AND characteristic).
The receiver converts the differential bus signal to a logic level signal, which is output at
RXD. The serial receive data stream is provided to the bus protocol controller for
decoding. The receiver comparator is always active i.e. it monitors the bus while the bus
node is transmitting a message. This is required to support the non-destructive bit-by-bit
arbitration scheme of CAN.
Single Ended
Bus Voltage
CANH
3.5V
2.5V
CANL
1.5V
Differential
Bus Voltage
5.0V
Differential input
voltage range for
dominant state
0.9V
Differential input
voltage range for
recessive state
0.5V
-1.0V
time
Recessive
Dominant
Recessive
Fig 3. Typical bus levels according to ISO11898-2
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TJA1041/1041A high speed CAN transceiver
VCC
VI/O
VBAT
INH
TXD
Time-Out
EN
Level
Adaptor
CANH
CANL
Driver
STBN
VBAT
VCC
Wake
Comparator
WAKE
Wake-up
+
Mode
Control
+
Failure
Detector
VI/O
ERRN
RXD
Recessive
Detection
VI/O
V Split
SPLIT
Low Power
Receiver
Normal
Receiver
RXD
Temp.
Protection
GND
Fig 4. Block diagram of the TJA1041/TJA1041A
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3. Target applications for TJA1041/TJA1041A
The TJA1041(A) high speed CAN transceiver is the ideal choice for applications which
require high data throughput (up to 1 Mbit/s), bus diagnosis and enhanced low-power
management. While the focus of the high speed CAN bus was mainly on Powertrain
applications, particularly the low-power management of the TJA1041 enables the
High-Speed CAN bus also for other in-vehicle multiplexing tasks like body multiplexing
and ITS data bus Ref. 10.
From an ECU power management point of view, four different application fields (type A to
D) can be distinguished for high speed CAN, as illustrated in Figure 5.
Clamp-30
BAT
VCC
Clamp-15
on/off
VCC
CTRL
μC
A
TXD
RXD
VCC
VCC
CTRL
μC
1041
TXD
RXD
B
μC
1040
C
TXD
RXD
1040
1050
μC
TXD
RXD
1050
1040
D
CANH
CANL
Fig 5. Target applications for the TJA1041(A) and TJA1040, TJA1050
Type A — Applications, which have to be available all time, even when the car is parked
and ignition-key is off, are permanently supplied from a permanent battery supply line,
often called “Clamp-30”. However, those nodes need the possibility to reduce the current
consumption for saving the battery by control of the local ECU supply (VCC). These type A
applications allow switching off the entire supply system of the ECU including the
microcontroller supply while keeping the wake-up capability via CAN.
The TJA1041(A) is the first choice for these applications. It can be put into its Sleep mode
(all VCC supplies off), which allows reducing the total current consumption of the entire
ECU down to typically 20 μA, while keeping the capability to receive wake-up events from
the bus and to restart the application.
Type B — Those applications, which need an always-active microcontroller, are
permanently supplied from the battery supply line “Clamp-30” using a continuously active
VCC supply. In order to reduce the ECU power consumption, the transceiver needs to be
set into a mode with reduced supply current while the VCC stays active.
Here the Standby mode of the TJA1040 offers the best choice. During Standby mode the
device reduces the VCC supply current to a minimum, while still monitoring the CAN bus
lines for bus traffic.
Type C — Dedicated applications, which need an always-active microcontroller and
therefore are permanently supplied from the Clamp-30 line, additionally come with a
microcontroller controlled transceiver voltage supply. In contrast to type B applications,
further current can be saved, because the transceiver becomes completely un-powered
by microcontroller control. These applications require absolute passive bus behavior of
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the transceiver, while its voltage supply is inactive. This is important in order not to affect
the remaining bus system, which might still continue communication. Most suitable for
such kind of applications are the TJA1040 and TJA1050. While the TJA1050 allows some
reverse current from the bus when becoming un-powered, the TJA1040 behaves
absolutely passive to the bus.
Type D — Applications, which do not need to be available with ignition-key off, are simply
switched off and become totally un-powered during ignition-key off. They are supplied
from a switched battery supply line, often called “Clamp-15”. This supply line is only
switched on with ignition-key on. Depending on system requirements, e.g. partial
communication of the still supplied nodes during “ignition-key off”, these un-powered
nodes need to behave passively towards the remaining bus, similar to type C applications.
As for type C applications, it is recommended to use the TJA1040 or TJA1050 due to its
passive behavior to the bus when becoming un-powered.
4. Main features
Today bus implementations call increasingly for low system power consumption, high
system reliability, excellent EMC (Electromagnetic Compatibility) and flexible interfacing.
The new features of the TJA1041(A), shown in Figure 6, reflect this increasing demand on
bus transceiver.
Very Low Power
System Fail-Safe
Features
Bus Failure
Diagnosis
TJA1041
Common Mode
Stabilization
I/O Level
Adaptation to μC
Fig 6. The new features of the TJA1041
4.1 Low power management
Modern in-vehicle networking architectures require the availability of the high speed CAN
bus even when ignition key is off. This requires permanently battery supplied ECUs with
lowest current consumption. The low power management of the TJA1041(A) allows
reducing the quiescent current consumption of an ECU to about typ. 20 μA. This current
consumption is low enough to allow permanent battery supply of the transceiver and
keeping wakeup capability via the bus. This way the system can react on local events as
well as on CAN messages, resulting in wakeup of the complete bus system.
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The operating modes of the TJA1041(A) (Normal, Pwon/Listen-Only, Standby, Sleep,
Goto-Sleep Command) establish a low power management with three different levels as
sketched in Figure 7 and Table 1. In level 0 the ECU components (voltage regulator,
microcontroller, transceiver, peripherals) are active and powered. The TJA1041(A) is
either in Normal or Pwon/Listen-Only mode. The transceiver and the host microcontroller
are powered by the active VCC supply.
The next level of low power, level 1, is achieved with the TJA1041(A) operating in Standby
mode. The microcontroller, transceiver and peripherals are still powered by the active VCC
supply, but the functionality is often reduced to a minimum in order to save current. In the
case of the TJA1041(A) the function is reduced to detection of wakeup events only.
Transmit and receive function as provided in Normal mode is not available. The host
microcontroller is often put in a power-down condition in order to save additional current.
The low power level 2 is associated to the Sleep mode of the TJA1041(A). In Sleep mode
the external voltage regulator(s), supplying the transceiver and host microcontroller, is
(are) typically switched off via the INH output signal of the transceiver. The VCC supply for
the transceiver and microcontroller is not available. While the host microcontroller and
peripherals are completely un-powered, the TJA1041(A) keeps powered via the battery
supply pin VBAT. This supply is needed to ensure wakeup capability either via the bus or
via a local wakeup event. The low power level 2 guarantees the lowest current
consumption of a node.
Table 1.
Characteristics of the different low power levels
Low power level
Operating mode
VCC supply
μC
Node power
consumption
Level 0
(Bus active)
Normal,
Pwon/Listen-Only
Active
Powered
Normal
Level 1
Standby
Active
Powered
Low
Level 2
Sleep
Off
Un-powered
Very low
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INH: HIGH
BAT
VCC on
Voltage
Regulator
STB=1
CANH
μC
+
CAN
EN=1/0
1041
CANL
Periph
eral
GND
A) TJA1041(A) in Normal/Pwon-Listen-Only Mode (Low-power level 0)
μC and peripheral are powered by VCC and active
INH: HIGH
BAT
VCC on
Voltage
Regulator
STB=0
CANH
μC
+
CAN
EN=0
1041
CANL
Periph
eral
GND
B) TJA1041(A) in Standby Mode (Low-power level 1)
μC and peripheral are powered by VCC, but may be in power-down mode
INH: LOW
BAT
VCC off
Voltage
Regulator
STB=0
CANH
CANL
EN=1
1041
μC
+
CAN
Periph
eral
GND
C) TJA1041(A) in Sleep Mode (Low-power level 2)
μC and peripheral are typically un-powered
Fig 7. Low power management of TJA1041(A)
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4.2 Bus failure diagnosis
The TJA1041(A) can detect short circuits on the bus wires and signal them to the host
microcontroller. While physical bus failures normally lead to interruption of bus
communication, there are certain bus failures that are tolerated within the physical layer of
high speed CAN. Without the bus failure diagnosis feature of the TJA1041(A) the
application microcontroller would not have a chance to become aware of those bus
failures. Apart from increasing current consumption, those bus failures are responsible for
poor EMC performance.
4.3 System fail-safe features
The system fail-safe features of the TJA1041(A) aim to keep the impact of possible local
failures, like pin short-circuits, confined to the corrupted node only. After detection of a
local failure, appropriate measures are taken to keep the remaining bus operable as long
as possible. There are protections against TXD Dominant Clamping, TXD/RXD Short
Circuit, RXD Recessive Clamping and Bus Dominant Clamping.
4.4 Common mode stabilization
The TJA1041(A) provides means for common mode stabilization by offering a
low-impedance voltage source of nominal VCC/2 at the pin SPLIT. This feature helps to
stabilize the bus common mode voltage, especially with leakage currents on the bus.
Such leakage currents may occur in presence of un-powered nodes during ignition-off. As
a result this feature can improve the EMC performance in presence of unpowered ECUs.
(see also Section 6.1.11)
4.5 I/O level adaptation to host controller supply voltage
The I/O level adaptation of the TJA1041(A) allows the transceiver to be interfaced directly
to any microcontroller with a typical supply voltage between 2.8V and 5V, without need for
extra glue logic between transceiver and microcontroller.
The TJA1041(A) provides a continuous threshold level adaptation for the interface pins to
the microcontroller down to a microcontroller supply voltage of 2.8 V Ref. 1. For this
purpose the host controller supply voltage is connected to the transceiver pin VIO to
provide the reference voltage for the input/output interface. It defines the ratiometric digital
input threshold for TXD, EN and STBN and the HIGH-level output voltage for RXD and
ERRN.
5. Operating modes
The TJA1041(A) provides five different operating modes, which are controlled by the input
pins STBN and EN. The reference state diagram for the operating modes can be found in
the data sheet Ref. 1. In the case of an undervoltage condition on the pin VCC or VIO , the
transceiver is forced into Sleep mode, overruling the current mode selection at the pins
STBN and EN. In the case of an undervoltage condition on the pin VBAT the transceiver is
forced into Standby mode.
Depending on the operating mode the transceiver shows different behavior for the
receiver and bus driver as well as on output pins like ERRN and RXD. Table 1
summarizes the characteristics in each operating mode.
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5.1 Normal mode
For CAN communication the Normal mode is chosen. The digital bit stream input at TXD
is transferred into corresponding analog bus signals. Simultaneously, the transceiver
monitors the bus, converting the analog bus signals into the corresponding digital bit
stream output at RXD. The external voltage regulator is active, the bus lines are biased to
VCC/2 and the transmitter is enabled. The Normal mode is entered setting STBN to HIGH
and EN to HIGH level.
5.2 Pwon/Listen-only mode
In general the Pwon/Listen-Only mode has two different functions. First, it realizes a
Listen-Only behavior. The node is only allowed to receive messages from the bus but not
to transmit onto the bus. The digital bit stream from the CAN-controller at TXD is simply
ignored. In this way a node can be prevented from influencing the bus.
Secondly, the Pwon/Listen-Only mode provides the Local Failure flag and PWON flag at
the pin ERRN, which can be read by the microcontroller. For flag signalling at the pin
ERRN refer to Section 7. The Pwon/Listen-Only mode is entered setting STBN to HIGH
and EN to LOW level.
5.3 Standby mode
Standby mode is used to achieve low power level 1. Power consumption of the
TJA1041(A) is significantly reduced compared to Normal or Pwon/Listen-Only mode. In
Standby mode the TJA1041(A) is not capable of transmitting and receiving regular CAN
messages. However, the TJA1041(A) monitors the bus for CAN messages. Whenever a
wakeup pattern is detected on the bus, indicating bus traffic, the internal wakeup flag is
set. The TJA1041(A) can also receive a local wakeup via the pin WAKE. On detection of a
remote or local wakeup the internal Wakeup flag is set. In Standby mode this flag is output
at the pins ERRN and RXD. To reduce the current consumption as far as possible the bus
is terminated to GND rather than biased to VCC/2 as in Normal or Pwon/Listen-Only mode.
Standby mode is selected setting STBN to LOW and EN to LOW level.
5.4 Sleep mode
Sleep mode is used to achieve low power level 2. While the transceiver current
consumption is the same as in Standby mode, it allows further reduction of the system
current consumption by switching off the external voltage regulator (VCC supply) for the
transceiver, host microcontroller etc.
The only difference between Sleep and Standby mode concerns the pin INH. It provides a
battery related open drain output to control one or more external voltage regulators. In
Sleep mode the pin INH is set floating compared to a HIGH signal (VBAT-based) in all
other modes (also Standby mode), typically disabling the voltage regulator(s) for the
transceiver and microcontroller. While the microcontroller is completely un-powered (no
VCC supply), the TJA1041(A) keeps partly alive via its battery supply. It allows the
transceiver to monitor the bus for CAN messages. In fact, the transceiver is the device
controlling autonomously the VCC supply for the ECU.
Wakeup from Sleep mode is generally possible via two channels:
• Wakeup via a dominant bus state
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• Local wakeup via an edge at pin WAKE
On wakeup, the pin INH goes HIGH enabling the external voltage regulator(s) again. The
wakeup flag is set for a local or remote wakeup. It is reflected at the pins ERRN and RXD.
As in Standby mode, the bus lines CANH and CANL are terminated to GND. Table 2
summarizes the characteristics of the TJA1041(A) in the different operating modes.
The only way to put the TJA1041(A) into Sleep mode is using the Goto Sleep Command
mode (STBN to LOW, EN to HIGH). If it is selected for longer than the minimum hold time
of go-to-sleep command th(min) Ref. 1, the transceiver is automatically forced into Sleep
mode switching the pin INH to floating.
A mode transition from Sleep mode to any other mode via STBN and EN is possible only
if the supply voltage VCC and VIO were present all time during Sleep mode. Once a VCC or
VIO under-voltage is detected, mode control via STBN and EN is disabled for fail-safe
reasons. This feature prevents the microcontroller from continuously waking up the
transceiver via STBN and EN in the case of a VCC or VIO under-voltage being detected by
the transceiver.
A continuous situation, where one part of the nodes is in Normal or Pwon/Listen-Only
mode while the other part is in Standby or Sleep mode, should be avoided due to the
different bus biasing in these modes. Otherwise a continuous DC common mode current
would flow from one part to the other.
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5.5 Go-to Sleep Command mode
The Go-to-Sleep Command mode has the meaning of a command rather than the
meaning of a typical operating mode. It is used to put the TJA1041(A) into Sleep mode.
Due to the spread of the minimum hold time of go-to-sleep command th(min) Ref. 1 the
Go-to Sleep Command mode must be actually selected for longer than the maximum
value in order to make sure the Sleep mode is entered reliably. Immediately after selecting
the Go-to Sleep Command mode the transmitter is disabled, the bus lines are terminated
to GND and the Wakeup flag information is signalled at the pins ERRN and RXD. The
Go-to Sleep Command mode is selected with STBN to LOW and EN to HIGH level.
Remark: the Go-to Sleep Command might become overruled by a wake-up event, if this
wake-up event occurs simultaneously with the Go-to Sleep Command. In this case, the
wake-up will be signalled on RXD and ERRN as desired, while INH stays active HIGH.
Table 2.
Characteristics of the different modes
Operating STBN
mode
EN
Normal
1
1
Pwon/
1
Listen-only
0
ERRN-pin
RXD-pin
Bus
bias
INHpin
Low
High
Low
High
Bus Failure
flag set [1]
Bus Failure
flag reset [1]
Bus
dominant
Bus
recessive
VCC/2 VBAT
Wakeup
Source flag;
local wakeup
detected [2]
Wakeup
Source flag;
remote
wakeup
detected [2]
PWON flag
set;
PWON flag
reset; [3]
Bus
dominant
Bus
recessive
VCC/2 VBAT
Wakeup
flag set [5]
Wakeup
flag reset
GND
[3]
Local Failure
flag set [4]
Local Failure
flag reset [4]
Wakeup flag
set [5]
Wakeup flag
reset [5]
Go-to
0
Sleep
Command
1
VBAT
Standby
0
0
VBAT
Sleep;
0
X
float
[6]
[1]
Valid after the fourth dominant to recessive edge at TXD after entering the Normal mode (each dominant
period should be at least 4μs).
[2]
Valid before the fourth dominant to recessive edges at TXD after entering the Normal mode.
[3]
Valid if VIO and VCC are present and coming from Sleep, Standby or Go-To-Sleep Command mode.
[4]
Valid if VIO and VCC are present and coming from Normal mode.
[5]
Valid if VIO and VCC are present .
[6]
Transceiver will enter the Sleep mode only if the Go-To-Sleep Command mode was selected longer than
the hold time of go-to-sleep command (th(min)) or by an under-voltage detection on VIO or VCC.
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6. Hardware application of the TJA1041(A)
Figure 8 shows how to integrate the TJA1041(A) within a typical application. The
application example assumes a 3.3 V supplied host microcontroller. There is a dedicated
5 V regulator supplying the TJA1041(A) transceiver and a dedicated 3.3 V regulator
supplying the microcontroller. Both voltage regulators are controlled via the INH output of
the transceiver, so that in Sleep mode both voltage regulators are switched off.
Furthermore, the application example makes use of the pin WAKE for local wakeup
possibility, connecting it to a low-side switch.
**
optional *
VCC
uC
+
CAN
**
TXD
TXD
RXD
RXD
VI/O
VCC
BAT
VCC
uC
5V
CAN
100n
INH
STB
I/O
I/O
EN
I/O
ERR
VBAT
1k
GND
10n
<150k
TJA1041
Rexbias
CANH
60 (1k3)***
RS
<100pF
WAKE
3k
SPLIT
60 (1k3)***
WAKE-UP
4n7
CANL
GND
<100pF
CAN
bus
*
For further EMC optimization a series resistor could be applied in case the bus timing parameters allow this additional delay
caused by the additional R/C time constant.
** Size of capacitor depends on regulator.
*** For stub nodes a "weak" termination improves the EMC behaviour of the system in terms of emission.
Fig 8. Typical application for 3.3 V microcontroller
6.1 Pin description
6.1.1 Pin VBAT
The battery supply ensures the local and remote wakeup capability of the TJA1041(A)
when the VCC supply is switched off during Sleep mode. Nevertheless the current
consumption IBAT via this pin is very low Ref. 1. It is recommended to place a series
resistor of 1 kΩ into the battery supply line of the transceiver for enhanced protection
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against automotive transients. Given the max. supply current IBAT of 40 μA at VBAT, a
voltage drop of 40 mV must be taken into account when calculating the minimum battery
operating voltage. In addition, a capacitor of about 10 nF, closely connected to the VBAT
pin and forming a low-pass filter in conjunction with the series resistor, can be used for
enhanced transient protection.
6.1.2 Pin VCC
The VCC supply provides the current needed for the transmitter and receiver of the
TJA1041(A). The VCC supply must be able to deliver current of 65 mA in average for the
transceiver (see Section 15.2). Using a linear voltage regulator, it is recommended to
stabilize the output voltage with a bypass capacitor of about 20 μF (see Section 15.2).
This type of capacitor should be connected at the output of the voltage regulator. In
addition, a second capacitor should be connected as close as possible between pin VCC
and GND of the transceiver. Its function is to buffer the VCC supply voltage, especially
during fast load changes at a transition from recessive to dominant. Recommended value
is 47-100 nF. For reliability reasons it might be useful to apply two capacitors in series
connection between VCC and GND. A single shorted capacitor (e.g. damaged device)
cannot short-circuit the VCC supply.
6.1.3 Pin VIO
Pin VIO is connected to the μC supply voltage to provide the proper voltage reference for
the input threshold of digital input pins and for the HIGH voltage of digital outputs. Unlike
other products on the market, the TJA1041(A) provides a continuous level adaptation
from as low as 2.8 V to 5 V. The level adaptation applies to all interface pins between the
microcontroller and the transceiver, i.e. TXD, EN, STBN (input pins) and RXD, ERRN
(output pins). In Normal mode only a small current IIO Ref. 1 is drawn out of the VIO
supply. In Sleep mode no current will be drawn via this pin. If the pin VIO is disconnected,
an under-voltage condition on VIO will be detected and the transceiver is forced into Sleep
mode in order to provide defined fail-safe low-power system behavior.
6.1.4 Pin TXD
The transceiver receives the digital bit stream to be transmitted onto the bus via the pin
TXD. When applied signals at TXD show fast slopes, it may cause a degradation of the
EMC performance. In this case, it is recommended to place a series resistor of about 1 kΩ
into the TXD line between transceiver and microcontroller. Along with pin capacitance this
would help to smooth the edges to some degree. For high bus speeds (close to 1 Mbit/s)
the additional delay within TXD has to be taken into account.
6.1.5 Pin RXD
The analog bit stream received from the bus is output at pin RXD for further processing
within the CAN-controller. As with pin TXD a series resistor of about 1 kΩ can be used to
smooth the edges at bit transitions. Again the additional delay within RXD has to be taken
into account, if high bus speeds close to 1 Mbit/s are used.
6.1.6 Pin STBN/EN
The pins STBN and EN are used for mode control. They are typically connected to an
output port pin of a microcontroller.
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6.1.7 Pin ERRN
The pin ERRN is a push-pull output stage for signalling failure conditions to the
microcontroller. It is typically connected to an input port pin of a microcontroller. (see
Ref. 1 for drive capability)
6.1.8 Pin CANH/L
The transceiver is connected to the bus via pin CANH/L. Nodes connected to the bus end
must show a differential termination, which is approximately equal to the characteristic
impedance of the bus line in order to suppress signal reflection. Instead of a one-resistor
termination it is highly recommended using the Split Termination, illustrated in Figure 9.
EMC measurements have shown that the Split Termination can improve significantly the
signal symmetry between CANH and CANL, reducing emission. Basically each of the two
termination resistors is split into two resistors of equal value, i.e. two resistors of 60 Ω
instead of one resistor of 120 Ω. The special characteristic of this approach is that the
common mode signal, available at the centre tap of the termination, is terminated to
ground via a capacitor. The recommended value for this capacitor is in the range of 4.7 nF
and 47 nF.
As the symmetry of the two signal lines is crucial for the emission performance of the
system, the matching tolerance of the two termination resistors should be as low as
possible (desired: < 2 %).
Also depicted in Figure 9, it is recommended to load the CANH and CANL pin each with a
capacitor of about 100 pF close to the connector of the ECU. The main reason is to
increase the robustness to automotive transients and ESD. The matching tolerance of the
two capacitors should be as low as possible.
6.1.9 Pin SPLIT
In Normal and Pwon/Listen-Only mode the pin SPLIT provides an output voltage of VCC/2.
In all other modes the pin is in high-ohmic state. By simply connecting the pin SPLIT to the
center tap of the Split Termination as shown in Figure 9, DC stabilization of the common
mode voltage is achieved.
In the case of unpowered nodes, leakage currents from the bus into the transceiver may
force the common mode voltage to drop below VCC/2 during recessive state. The DC
stabilization aims to oppose this degradation and helps improving the emission
performance. With no significant leakage currents from the bus, the pin SPLIT can be left
open.
According to the data sheet Ref. 1 the maximum impedance of the voltage source can be
calculated to 2000 Ω.
6.1.10 Pin WAKE
The pin WAKE can be used to signal a local wakeup event to the transceiver. Like the
Fault-tolerant CAN Transceiver TJA1054 Ref. 5 a signal change of sufficient length at the
pin WAKE generates a local wakeup. That means both a rising and a falling edge can be
applied to launch a local wakeup. Typically, a low-side switch is used at the pin WAKE as
shown in Figure 9. The series resistor RS is for protection and limits the output current
when the transceiver has lost its ground connection, while the low-side switch is closed.
The pull-up resistor Rexbias is needed to provide a sufficient current for the switch contacts.
More information on the values for RS and Rexbias is given in Section 7.2.1.
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6.1.11 Pin INH
The intention of the pin INH is to control one or more voltage regulators within the ECU. In
Figure 9 two voltage regulators, one 5 V regulator for the transceiver and one 3.3 V
regulator for the microcontroller, are controlled via the INH output of the transceiver as an
example.
The pin INH provides a battery related open drain output. During Sleep mode it is floating.
Due to the typical pull-down behavior of the Inhibit input pin of common voltage regulators,
this results in a LOW signal on the Inhibit input, typically disabling the voltage regulator(s).
In all other operation modes the pin INH is actively pulled to battery voltage, enabling the
external voltage regulator(s). The load resistance at the pin INH shall not be lower than
10 kΩ for 12 V battery systems. If the pin INH is not used for voltage regulator control, it
can be left open.
6.2 Application of common mode choke
A common mode choke provides high impedance for common mode signals and low
impedance for differential data signals. Due to this, common mode signals, either
produced by RF noise or by the transceiver itself, get effectively attenuated while passing
the choke. In fact, a common mode choke helps to reduce emission and to improve
immunity against common mode disturbances.
Former transceiver devices usually needed a common mode choke to fulfill the stringent
emission and immunity requirements of automotive when using unshielded twisted-pair
cable. The TJA1041(A) has the potential to build in-vehicle bus systems without chokes.
Whether a choke is needed or not finally depends on the specific system implementation
like the wiring harness and the symmetry of the two bus lines (matching tolerances of
resistors and capacitors).
0Ω
SPLIT
CANH
RT/2
CAN
bus
TJA1041
RT/2
CSPLIT
CANL
CH
Split
Common
Capacitors
Mode Choke Termination (Optional)
(i.e. B82789-C104)
CL
ESD protection diodes
(e.g. NXP PESD1CAN)
Fig 9. Application with choke and ESD protection
Besides RF noise reduction, stray inductance (non-coupled portion of inductance) may
establish a resonant circuit together with pin capacitance. This can result in unwanted
resonance oscillations between the bus pins and the choke, both for differential and
common mode signals, and in extra emission around the resonant frequency. To avoid
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such oscillations, it is highly recommended to use only chokes with a stray inductance
lower than 500 nH. Bifilar wound chokes typically show an even lower stray inductance.
As shown in Figure 9 the choke should be placed near to the transceiver.
6.3 ESD protection
The TJA1041(A) is designed to withstand ESD pulses of up to 6 kV according to the
human body model at the bus pins CANH, CANL and pin SPLIT and typically does not
need further external protection methods. Nevertheless, if much higher protection is
required, external clamping devices can be applied to the CANH and CANL line.
NXP offers a dedicated protection device for the CAN bus, providing high robustness
against ESD and automotive transients. The PESD1CAN ESD protection diode Ref. 13,
featuring a very fast diode structure with very low capacitance (typ. 11 pF), is compliant
with IEC61000-4-2 (level 4), allowing air and contact discharge of more than 15 kV and
8 kV, respectively. Tests at an independent test house have confirmed more than 20 kV
ESD robustness for ECUs equipped with the PESD1CAN diode and a choke. To be most
effective the PESD1CAN diode should be placed close to the ECU connector as shown in
Figure 9 while the choke should be placed near to the transceiver.
6.4 PCB layout rules
Following guidelines should be considered for the PCB layout.
• When a common mode choke is used, it should be placed close to the transceiver bus
pins CANH and CANL.
• The PCB tracks for the bus signals CANH and CANL should be routed close together
in a symmetrical way. Its length should not exceed 10 cm.
• Suppressor diodes or varistors for ESD protection should be connected close to the
ECU connector bus terminals.
7. Wakeup detection
There are in general two possibilities to wake up the transceiver, either via the bus or via
the WAKE pin. On detection of a wakeup event the Wakeup flag is set and signalled at
RXD and ERRN.
7.1 Wakeup via bus
The only difference between the TJA1041 and TJA1041A concerns the bus wakeup
mechanism. The TJA1041 detects a bus wake-up request when the bus remains in
dominant state for at least 5 µs. In contrast to that, the TJA1041A detects a bus wake-up
request when the bus shows two dominant phases of at least 5 µs duration, with each
dominant phase followed by a recessive phase of at least 5 µs. Figure 10 illustrates the
bus wake-up pattern requirements for the TJA1041 and TJA1041A.
The enhanced bus wakeup detection offers improved robustness against unwanted
wakeup in presence of bus failures, especially for large networks. There will be no
unwanted bus wake-up due to a BAT-to-CANH short-circuit or CANL wire interruption,
while the system is entering Bus Sleep.
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At a data rate of 500 kbit/s, a single arbitrary CAN data frame is not necessarily sufficient
to launch a remote wake-up with the enhanced wake-up pattern. Here, two consecutive
arbitrary CAN data frames are needed to reliably launch a remote wake-up. At 125 kbit/s
data rate, any CAN data frame on the bus will lead to a remote wake-up of the TJA1041A
transceiver.
dominant
bus wake-up pattern for
TJA1041
Vdiff,bus
> tBUS
dominant
recessive
dominant
recessive
bus wake-up pattern for
TJA1041A
Vdiff,bus
> tBUS
> tBUS
> tBUS
> tBUS
(tBUS : 0,75...5us)
Fig 10. Bus wakeup pattern for TJA1041 and TJA1041A
7.2 Wakeup via WAKE pin
The pin WAKE can be used to signal a local wakeup event to the transceiver. Like the
Fault-tolerant CAN Transceiver TJA1054 Ref. 5 a signal change of sufficient length at the
pin WAKE generates a local wakeup. While the TJA1054 features an internal pull-up to
battery voltage to allow use of a low-side switch, the pin WAKE of the TJA1041(A)
features variable biasing. Depending on the external biasing the internal one switches
from GND to battery level or vice versa. Figure 11 illustrates the biasing concept of the pin
WAKE along with different external switching circuits.
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BAT
Rexbias
0 1: 5-50μs
1 0: 5-50μs
Wake
Timer
RS
Vth
Rexbias
GND
Example A
Example B
Example C
Transceiver
Fig 11. External switching circuits for the pin WAKE
If a voltage higher than the Wakeup Threshold Voltage Vth(WAKE) Ref. 1 is held at the pin
WAKE for longer than the maximum time tWAKE Ref. 1, the internal biasing (current
source) will switch reliably to battery level if the pin was at LOW level before. Similarly, if a
voltage lower than this value is held for longer than the max. tWAKE time, the internal
biasing (current source) switches reliably to GND if the pin was at HIGH level before. The
internal biasing is adapted automatically to the external biasing conditions. This concept
allows using a low-side switch as well as a high-side switch or a VBAT based push-pull
stage without forcing undesired bias currents while there is no wakeup event.
In the case of a low-side switch, both the resistor Rexbias and the internal current source
provide a pull-up to VBAT. In order to launch a local wakeup the external switch has to be
closed producing a negative pulse at the pin WAKE. The negative pulse passes the
internal timer and releases a wakeup reliably if the pulse is longer than the maximum
value of tWAKE Ref. 1. Along with passing the timer the bias switches to GND. After
releasing the low-side switch the external pull-up resistor switches the internal bias back
to VBAT. The resistor Rexbias determines the current through the external switch when it is
closed and is needed to guarantee a proper switch contact.
If pin WAKE is not used, connect the pin directly to ground level.
7.2.1 Dimensioning of RS and Rexbias
The purpose of the series resistor RS is to protect the transceiver if the ECU has a loss of
ground situation while the wakeup switch still is connected to a proper GND. The
minimum required series resistor is determined by the expected maximum battery supply
voltage VBAT,max and the maximum allowed current at pin WAKE of 15 mA. The resistor
should make sure that the current does never exceed this level. The minimum required
series resistor RS can be calculated by:
V BAT,max
R S,min = -------------------I Wake,max
(1)
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Assuming that VBAT will not exceed 40 V DC, the series resistor should have a value of
3 kΩ.
The resistor Rexbias is needed to turn the bias to its default state after the external switch
has been released. That defines an upper limit for the resistor value. For example, with a
low-side switch the resistor Rexbias together with the series resistor RS must pull the pin
WAKE above the switching threshold of the pin WAKE. The equation for determining the
upper limit for Rexbias is:
( R exbias + R S ) ⋅ I Pull,max < V BAT – V th(Wake),max
(2)
With the maximum pull-down (pull-up) current of 10 mA and the maximum threshold of
Vth(WAKE), the theoretical upper limit for Rexbias calculates to about 180 kΩ. A typical value
is 20 kΩ.
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8. Flag signaling
The TJA1041(A) provides five different flags to be signalled to the microcontroller. The
status of the flags can be read by the microcontroller via the pin ERRN. Which flag is
actually signalled on the pin ERRN depends on the current operating mode and on the
history. Figure 12 shows the flag signaling of the pin ERRN.
Notice that when switching from one mode to another, it takes some time until the “new”
flag is signaled at ERRN. To read pin ERRN with the application software, after a mode
transition has been performed, first introduce a wait time in the software of at least 10 μs.
8.1 Wakeup flag
A wakeup event from the bus or the pin WAKE sets the Wakeup flag only if the wakeup
event is received while the TJA1041(A) is in Sleep, Standby or Go-to Sleep Command
mode. In Normal or Pwon/Listen-Only mode any wakeup event is ignored. The Wakeup
flag is signalled at the pin ERRN during Sleep, Standby and Go-to Sleep Command mode
provided that VCC and VIO are present. A LOW level signals a wakeup request for the
microcontroller, received either via the bus or via the pin WAKE. It is reset either when the
Normal mode is entered or when there was a BAT-Under-voltage condition detected. As
long as the Wakeup flag is set, a Go-to Sleep Command is simply ignored and a transition
into Sleep mode is not possible. The Wakeup flag is also signalled at the pin RXD with the
same polarity as described for the pin ERRN.
8.2 PWON flag
The PWON flag is signalled at the pin ERRN during Pwon/Listen-Only mode when coming
from Standby, Sleep or Go-to Sleep Command mode. It is set to LOW level if there was a
battery under-voltage condition. If the battery has been connected the first time to the pin
VBAT or if there was a temporary battery under-voltage condition, this flag is set. The
PWON flag is reset once the Normal mode is entered. As long as the PWON flag is set, it
is not possible to enter Sleep mode. After first battery supply the PWON flag is initialized
to “set”, indicated by a LOW level at pin ERRN.
8.3 Wakeup source flag
Entering Normal mode, pin ERRN first reflects the Wakeup Source flag. A LOW level
signals a local wakeup via the pin WAKE, whereas a HIGH level indicates a wakeup via
the bus. The Wakeup Source flag is overwritten by the Bus Failure flag after the node has
transmitted four recessive-to-dominant bit transitions in Normal mode. Since the
application is controlling its own transmission behavior, the application has any time
needed to read this Wakeup Source flag. The Wakeup Source flag is cleared and set to
the default state HIGH whenever the Normal mode is left. After first battery supply the
Wakeup-Source flag is initialized to “wakeup via bus”, indicated by HIGH level at pin
ERRN.
8.4 Bus failure flag
After the transceiver has transmitted four recessive-to-dominant bit transitions with a
dominant bit length of at least 4 ms , the Bus Failure flag overwrites the Wakeup Source
flag. A LOW level indicates a short circuit condition on the bus. The Bus Failure flag is
reset to default HIGH on leaving Normal mode. If there is no local wakeup meantime,
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leaving and re-entering Normal mode forces pin ERRN to default state HIGH. Signalling
the Bus Failure flag requires retransmission of at least four recessive-to-dominant bit
transitions. Detection of bus failures does not lead to a change of transceiver operation.
Active fault tolerance as known from the TJA1054 low speed CAN transceiver is not
supported.
8.5 Local failure flag
Entering Pwon/Listen-Only mode from Normal mode, pin ERRN signals the Local Failure
flag. A LOW level indicates those failures, which are associated with the local node only
like
•
•
•
•
•
TXD Dominant Clamping
TXD/RXD Short Circuit
RXD Recessive Clamping
Bus Dominant Clamping
Over-temperature Condition
For a detailed description of the detected local failures refer to Section 10. If any of these
local failures is present, this is indicated to the application by an active LOW signal. A
more differentiated diagnosis is not supported. Along with setting the Local Failure flag,
the transmitter is disabled because of fail-safe reasons, except for RXD Dominant
Clamping detection. The Local Failure flag is reset and the transmitter enabled again
either by forcing a transition into Normal mode or by receiving a dominant bit from the bus
while TXD is recessive.
Normal
AND
4 dominant
periods
Bus
Failure
Flag
STB=1
EN->0
c
ag
(F l
(F lag c leared)
Normal
Mode
STB=1
EN->0
Local
Failure
Flag
Pwon
Listen
Wake-up
Source
Flag
)
red
l ea
STB=1
EN->1
PWON
Flag
STB->1
EN->1
STB->0
EN=X
Goto Sleep
Stby / Sleep
STB->0
EN=X
STB->0
EN=X
STB->1
EN->0
Wake-up
Flag
STB->0
EN=X
Power
On
Fig 12. Flag signaling of the pin ERRN
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9. Bus failure diagnosis
9.1 List of signaled bus failures
Assuming a bus load of nominal 60 Ω the following bus failure conditions are detectable
by the TJA1041(A):
•
•
•
•
•
•
CANH x BAT (communication still possible, “hidden” bus failure)
CANH x VCC (communication still possible, “hidden” bus failure)
CANH x GND (communication not possible)
CANL x BAT (communication not possible)
CANL x VCC (communication not possible)
CANL x GND (communication still possible, “hidden” bus failure)
Listed short-circuits are reliably detected in the range 0 to 50 Ω. A short-circuit between
CANH and CANL or line interruption failures are not detected. For analyzing the bus the
node needs to actively transmit onto the bus. Before the Bus Failure flag becomes valid,
the node must have transmitted at least four dominant sequences onto the bus each of at
least 4 μs length.
As already mentioned in Section 4.2, the bus system performance suffers from hidden
bus failure conditions in terms of EMC. The hidden bus failures are a short-circuit
CANHxBAT, CANHxVCC and CANLxGND. They are normally tolerated by the CAN
High-Speed Physical Layer as long as the capacitive load on the bus is not too large,
otherwise dominant periods on the bus would lengthen at the expense of recessive
periods, possibly causing bit timing violations. Communication between nodes is still
possible. Without additional diagnosis on physical layer level the microcontroller has no
chance to get to know from those bus failures. The bus failure diagnosis aims to detect
such failure conditions and to signal them to the application microcontroller.
9.2 How to read the bus failure flag
The Bus Failure flag is actually signalled at the pin ERRN. When entering Normal mode,
pin ERRN first reflects the Wakeup-source flag. After four dominant periods of sufficient
length have been transmitted, the Bus Failure flag gets active at the pin ERRN.
During arbitration, when more than one node may transmit simultaneously the bus failure
measurement process may be distorted, resulting in unstable bus failure information. It is
recommended that reading the Bus Failure flag from the microcontroller should take place
at the end of the CAN frame only, e.g. within the transmit interrupt service routine. The
read process should be completed before the transceiver sends the next CAN message.
In order to be able to guarantee the four needed dominant periods each of more than 4 μs
length, a dedicated diagnosis message with appropriate payload may be helpful,
especially for high bit rates.
A possible flow diagram for the transmit interrupt service routine is shown in Figure 13. If
reading of the pin ERRN indicates a LOW signal, a hidden bus failure must be present,
because with bus failures, leading to complete corruption of communication, the transmit
interrupt service routine would never be reached.
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Transceiver in
Normal Mode
Bus Failure
flag
Start Transmit
Interrupt Routine
Read ERR
ERR="0"
No
Yes
"Hidden" Bus
Failure
Store
Bus Failure Info
End Transmit
Interrupt Routine
Fig 13. Flow diagram for the transmit interrupt service routine
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10. Local failure diagnosis
Local failures detected and signalled at the pin ERRN in Pwon/Listen-Only mode (when
coming from Normal mode) include:
•
•
•
•
TXD Dominant Clamping
TXD/RXD Short Circuit
RXD Recessive Clamping
Over-temperature
On detection of one of these local failures, the Local Failure flag is set and the transmitter
disabled. Failures listed here and information about Bus Dominant Clamping are not
stored with the Local Failure flag. They are indicated at pin ERRN during
Pwon/Listen-Only mode. No other measure is taken in the case of Bus Dominant
Clamping.
10.1 Recovery from local failures
Whenever the pin RXD becomes dominant while TXD is recessive, the Local Failure flag
is reset along with enabling the transmitter again. This indicates that a local failure like
TXD Dominant Clamping, TXD/RXD Short Circuit or RXD Recessive Clamping does not
exist. In Pwon/Listen-Only mode failure recovery is immediately reflected on the pin
ERRN going HIGH again.
Another way to reset the Local Failure flag and to enable the transmitter is forcing a
transition into Normal mode from any other mode. This reset option is necessary when
there is no bus traffic i.e. the pin RXD does not become dominant. In this case the
application microcontroller can force a transition to Normal mode after it has read the error
status in Pwon/Listen-Only mode. If the failure is still present, it is detected again,
disabling the transmitter. Alternatively, if the failure is cleared, normal operation is
resumed. A suggested flow diagram for handling communication failures is shown in
Figure 16.
10.2 TXD dominant clamping
This fail-safe feature is already known from the TJA1050 and described in the Application
Note AN00020 Ref. 10. It prevents an erroneous CAN-controller from clamping the bus to
dominant level by a continuously dominant TXD signal.
After a maximum allowable TXD dominant time tDOM(TXD) Ref. 1 the transmitter is
disabled. According to the CAN protocol Ref. 2 only a maximum of eleven successive
dominant bits are allowed on TXD (worst case of five successive dominant bits followed
immediately by an error frame). Along with the minimum allowable TXD dominant time,
this limits the minimum bit rate to 40 kbit/s.
10.3 TXD/RXD short circuit
Without the protection feature of the TJA1041(A), a TXD/RXD short circuit would result in
a dead-lock situation clamping the bus dominant. If for example the transceiver receives a
dominant signal, RXD outputs a dominant level. Because of the short circuit, TXD reflects
a dominant signal, retaining the dominant bus state. As a result TXD and the bus are
clamped continuously dominant. The resulting effect is the same as for the continuously
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clamped dominant TXD signal. The TXD dominant timeout interrupts the deadlock
situation by disabling the transmitter. The bus and also TXD become recessive again.
However, the failure scenario may still exist and with the next dominant signal on the bus
the described procedure will start again. Apparently, the TXD dominant timeout alone is
not sufficient to protect the bus from a local TXD/RXD short circuit.
tDOM
TXD
dominant
transmitter
disabled
transmitter
enabled
Bus: dominant
&
TxD: recessive
CANH
VDiff
BUS
CANL
received bit(s)
transmitted bits
time
Fig 14. TXD dominant timeout and recovery mechanism
The TJA1041(A) keeps the transmitter off after detection of a TXD dominant clamping
even if TXD gets released again. Failure recovery is performed first if the transceiver has
detected a dominant bus signal while TXD is recessive. This is a clear indication that the
TXD/RXD has short-circuited. Figure 14 illustrates the disabling and enabling of the
transmitter with respect to a TXD/RXD short circuit. This way a local TXD/RXD short
circuit will not disturb the communication of the remaining bus system.
10.4 RXD recessive clamping
If the pin RXD is shorted to VCC or VIO, the pin RXD is clamped to recessive signal. Using
a conventional transceiver such a node assumes that the bus is permanently in Idle State.
So it launches a message transmission on the bus whenever necessary, regardless of
other bus traffic. As a result the bus traffic is heavily disturbed.
The TJA1041(A) can detect a RXD Recessive Clamping situation whenever it receives a
dominant bus signal. On detection the transmitter is disabled immediately, so that the
corrupted, non-synchronized node is prevented from disturbing the remaining bus
traffic.The corrupted node is then excluded from communication. It can neither transmit
nor receive any message, whereas the remaining bus is unaffected.
10.5 Bus dominant clamping
In the case of a short circuit from CANH to BAT/VCC, the circuit for the Common mode
Stabilization may produce a differential voltage on the bus between CANH and CANL
even if there is no dominant transmitting node. This is illustrated in Figure 15. The
differential voltage can be high enough to represent a dominant signal (Vdiff > 0,9 V). The
result may be a permanently dominant clamped bus in the case of a short circuit from
CANH to BAT.
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Vbat
VCC
CANH
R T/2
Ip
Vdiff
RT/2
CANL
GND
Fig 15. Bus dominant clamping in the case of a short circuit CANH to BAT
The TJA1041(A) can detect and report a Bus Dominant Clamping situation. If the receiver
detects a bus dominant phase of longer than the bus dominant time out tDOM(bus) Ref. 1 ,
this is indicated at pin ERRN in Pwon/Listen-Only mode.
10.6 Over temperature protection
An over-temperature condition may occur either if the transceiver is operated in an
environment with high ambient temperature or if there is a short circuit condition on the
bus. To protect the transceiver from self-destruction the transmitter is disabled
automatically whenever the junction temperature exceeds the allowed limit Ref. 1. In
addition the Local Failure flag is set, which can be read at pin ERRN in Pwon/Listen-Only
mode.
After an over-temperature condition the transmitter of the TJA1041(A) is released if the
junction temperature is below the limit and if there is a transition into Normal mode or
reception of a dominant bus signal while TXD is recessive.
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11. Under-voltage detection
Table 3.
Supply undervoltage detection
Under-voltage on
Detection condition
Mode change to
VCC
VCC < VCC(sleep) for longer than
under-voltage detection time [1]
Sleep
VIO
VIO < VIO(SLEEP) for longer than
under-voltage detection time [1]
Sleep
VBAT
VBAT < VBAT(stb);
no timeout
Standby
11.1 VCC/VIO under-voltage detection
On detection of a VCC or VIO under-voltage condition the transceiver is forced
autonomously into Sleep mode overruling the current signal combination on pin STBN
and pin EN. As a result, pin INH becomes floating, disabling the voltage regulator(s).
An under-voltage condition may occur if pin VCC and/or pin VIO are disconnected or if
there is a short circuit from VCC or VIO to GND e.g. due to a broken capacitor. In the case
of a short circuit, disabling the voltage regulator prevents flow of high short-circuit current.
The under-voltage condition must hold at least the “undervoltage detection time on VCC
and VIO” before the transceiver is forced into Sleep mode. This time-out is needed to
suppress the under-voltage detection during ramping up of VCC/VIO, e.g. on wakeup from
Sleep mode. A wakeup event either received from the bus or the pin WAKE wakes up the
transceiver (INH switched on) along with trying to ramp up VCC and/or VIO again. If there is
still an under-voltage condition on VCC and/or VIO, the transceiver is forced into Sleep
mode again.
Notice there is no dedicated signal to inform the microcontroller about a VCC
under-voltage condition at the transceiver. However, the microcontroller can learn from a
transceiver under-voltage condition by evaluating the pin INH. Whenever a VCC or VIO
under-voltage has been detected by the transceiver, a pull-down resistor would pull the
pin INH to LOW level.
11.2 VBAT under-voltage detection
The TJA1041(A) monitors the battery supply voltage at the pin VBAT. If the battery supply
voltage falls below the BAT-under-voltage threshold Ref. 1, the transceiver enters
autonomously the Standby mode, overruling the mode control pins STBN and EN. An
under-voltage condition on the pin VBAT may occur for example, if the pin VBAT has been
disconnected, or temporarily during start of the engine (pulse 4 of ISO7637). The
BAT-under-voltage threshold is related to the VCC supply voltage. A BAT-under-voltage
condition is detected only if VBAT < VCC -1.4 V(typ.) and will be taken off again when VBAT
crosses the detection threshold VBAT(STANDBY) upwards, leaving mode control to pin STBN
and EN.
In addition, whenever VBAT falls below VBAT(Pwon) Ref. 1, the PWON flag is set. The
microcontroller has access to this flag via the pin ERRN when the Pwon/Listen-Only mode
is entered from Sleep, Standby or Go-to-Sleep Command mode. A transition into Normal
mode deletes the PWON flag. In this way the application microcontroller can know from a
temporary, local battery under-voltage condition.
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12. Pin FMEA
This chapter provides an FMEA (Failure Mode and Effect Analysis) for typical failure
situations, when dedicated pins of the TJA1041(A) are short-circuited to supply voltages
like VBAT, VCC, GND or to neighbored pins or simply left open. The individual failures are
classified, due to their corresponding effects on the transceiver and bus communication in
Table 4.
Table 4.
Class
Classification of failure effects
Effects
A
- Damage to transceiver
- Bus may be affected
B
- No damage to transceiver
- No bus communication possible
C
- No damage to transceiver
- Bus communication possible
- Corrupted node excluded from communication
D
- No damage to transceiver
- Bus communication possible
- Reduced functionality of transceiver
Table 5.
Pin
FMEA matrix for pin short-circuits to VBAT and VCC
Short to VBAT (12 V ... 40 V)
Class
Remarks
Remarks
(1) TXD
A
Limiting value exceeded
C
TXD clamped recessive
(2) GND
C
Node is left unpowered
C
VCC under-voltage
detected; TRX goes to
Sleep
(3) VCC
A
Limiting value exceeded
---
---
(4) RXD
A
Limiting value exceeded
C
RXD recessive clamping,
transmitter disabled
(5) VIO
A
Limiting value exceeded
C
μC may be damaged if
VCC>VIO
(6) EN
A
Limiting value exceeded
D
Standby and
Pwon/Listen-Only not
selectable
(7) INH
D
Voltage regulator keeps
permanently
D
Voltage regulator keeps
permanently on
(8) ERRN
A
Limiting value exceeded
D
No failure signaling to μC
(9) WAKE
D
Local wakeup not
possible
D
Local wakeup not possible
(10) VBAT
---
---
A
Limiting value exceeded
(11) SPLIT
D
Bus charged to BAT-level;
bit timing problems
possible
D
Bus charged to VCC level;
bit timing problems
possible
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Short to VCC (5 V)
Class
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Table 5.
Pin
FMEA matrix for pin short-circuits to VBAT and VCC …continued
Short to VBAT (12 V ... 40 V)
Class
Remarks
Short to VCC (5 V)
Class
Remarks
(12) CANL
B
No bus communication
B
No bus communication
(13) CANH
D
Degradation of EMC; bit
timing problem possible
D
Degradation of EMC; bit
timing problem possible
(14) STBN
A
Limiting value exceeded
D
Standby and Goto-Sleep
not selectable
Table 6.
FMEA matrix for pin short-circuits to GND and open
Pin
Short to GND
Class
Open
Remarks
Class
Remarks
(1) TXD
C
TXD dominant clamping;
transmitter disabled
C
TXD clamped recessive
(2) GND
---
---
C
Under-voltage detected; TRX
goes to Sleep
(3) VCC
C
VCC Under-voltage
detected; TRX goes to
Sleep
C
VCC Under-voltage detected;
TRX goes to Sleep
(4) RXD
C
RXD clamped dominant
C
μC likely to go into Bus-Off
(5) VIO
C
VIO Under-voltage detected;
TRX goes to Sleep
C
VIO under-voltage detected;
TRX goes to Sleep
(6) EN
C
Normal and Goto-Sleep not
selectable
C
Normal and Goto-Sleep not
selectable
(7) INH
C
Voltage regulator keeps
permanently off
C
Voltage regulator switched off
(8) ERRN
D
No failure signaling to μC
D
No failure signaling to μC
(9) WAKE
D
Local wakeup not possible
D
Local wakeup not possible
(10) VBAT
C
Node is left unpowered
C
VBAT under-voltage detected,
TRX goes to Standby
(11) SPLIT
D
Bus discharged to
GND-level; bit timing
problems possible
D
No DC common mode
stabilization
(12) CANL
D
Degradation of EMC, bit
timing problem possible
C
Transmitting not possible
(13) CANH
B
No bus communication
C
Transmitting not possible
(14) STBN
C
Normal and
Pwon/Listen-Only not
selectable
C
Normal and
Pwon/Listen-Only not
selectable
Table 7.
FMEA matrix for pin short-circuits between neighbored pins
Pin
Short to neighbored pin
Class Remark
TXD-GND
C
Transmitter disabled after TXD Dominant Timeout
GND-VCC
C
VCC under-voltage detected; transceiver goes to Sleep
VCC-RXD
C
RXD recessive clamping; transmitter disabled
RXD-VIO
C
RXD recessive clamping; transmitter disabled
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Table 7.
FMEA matrix for pin short-circuits between neighbored pins …continued
Pin
Short to neighbored pin
Class Remark
VIO-EN
D
Standby and Pwon/Listen-Only not selectable
EN-INH
D
Voltage regulator may switch off in Standby and Pwon/Listen-Only
ERRN-WAKE
D
Local wakeup not possible, damage to transceiver only with closed
high-side switch
WAKE-VBAT
D
Local wakeup not possible
VBAT-SPLIT
D
Bus charged to BAT-level
SPLIT-CANL
D
Degradation of EMC
CANL-CANH
B
No bus communication
CANH-STBN
D
Transceiver is not able to enter low-power mode if the bus is driven
dominant
13. Software issues
13.1 Software flow for handling communication failures
Figure 16 suggests a software flow for handling communication failures. Starting from
normal operation with the TJA1041(A) in Normal mode, the host microcontroller reads the
Bus Failure flag at the pin ERRN whenever a communication failure has been reported by
an error interrupt of the CAN-controller or by a missing transmit interrupt.
If the Bus Failure flag is set, the communication failure is likely to be caused by a bus
failure. After a defined time-out period a new transmission attempt is performed. After a
maximum number of transmission attempts have failed, an application appropriate
fall-back procedure must be activated. On the other hand, if the Bus Failure flag is not set,
the communication failure is likely to be caused by a local failure. In order to check for a
local failure condition, the transceiver is forced into Pwon/Listen-Only mode. If a local
failure is signalled (see Section 10), the application waits for recovery reading periodically
the Local Failure flag. If there was no recovery within a defined time-out period, one option
can be forcing a transition into Normal mode with releasing the transmitter. If the failure
still exists, detection will disable the transmitter due to fail-safe reasons.
Another option is to use a fall-back procedure. If reading the Local Failure flag signals that
the failure is recovered, the transceiver is put into Normal mode and normal operation
continues.
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Transceiver in
Normal Mode
Normal Operation
e.g. Transmit/Receive Error counter
exceeded limit value
Communication
Error detected
Bus Failure
Flag
Read ERR
Timeout
Yes
Bus Failure
ERR=0 ?
Store
Failure Info
No
Pwon/
Listen-Only
Set STB = 1
EN = 0
Transmission
Attempt
Number
exceeded ?
Wait 10us
No
Yes
Local Failure
Flag
Read ERR
Fall-back Procedure
Wait for
recovery
ERR=0 ?
Yes
Local Failure
Time-out
expired
No
Normal
Mode
Set STB = 1
EN = 1
optional:
"forced" recovery
Fall-back Procedure
Fig 16. Flow diagram for handling communication failures
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13.2 Software flow for an ECU cold start
The PWON flag of the TJA1041(A) indicates to the microcontroller whether a
microcontroller cold start was caused by a wakeup from Sleep mode or by a first battery
power application. This information is often needed for the application to initiate some
possible calibration procedures on first battery power application.
BAT, GND connected;
INH=High;
STB, EN held LOW
during Vcc ramping-up
Select
Pwon/Listen-Only Mode
Cold Start
Set STB = 1
EN = 0
optional,
if PWON Flag
is not of interest
Wait 10us
PWON Flag
0 : First BAT Application
1 : Wake-up from Sleep
Read ERR
ERR=0 ?
Yes
No
Set STB = 1
EN = 1
Select
Normal Mode
System Start-up
Procedure
Wait 10us
optional
Wake-up Source Flag
0 : Local Wake-up
1 : Wake-up via Bus
Set STB = 1
EN = 1
Read ERR
End of
Cold Start
Fig 17. Flow diagram for an ECU cold start
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The pin ERRN reflects the PWON flag when entering the Pwon/Listen-Only mode from
Standby, Sleep or Go-to-Sleep Command mode. Moreover, with a wakeup from Sleep
mode the TJA1041(A) provides information on the wakeup source. Entering the Normal
mode the pin ERRN reflects the Wakeup Source flag. A LOW signal indicates a local
wakeup via the pin WAKE, whereas a HIGH signal indicates a remote wakeup via the bus.
If battery power is applied for the first time, an internal hardware reset signal is given to
the transceiver for initialization. Subsequently the PWON flag is set and the pin INH is
pulled to VBAT, activating the voltage regulator(s) and ramping up the VCC supply. Along
with VCC the pins RXD and ERRN go to HIGH level. With ramping up VCC the
microcontroller comes up. As almost all microcontrollers feature a weak pull-down or
floating behavior at their port pins, the TJA1041 comes up in Standby mode after first
battery power application. This is the starting point for the application program taking over
the control now. If the microcontroller comes up with a HIGH level at its port pins, the
TJA1041(A) enters immediately the Normal mode and the PWON flag information is
irretrievably lost. Figure 17 suggests a software flow for an ECU cold start. It considers
primarily the issues related to the TJA1041(A) rather than representing a complete
software flow. After the transceiver and microcontroller have performed their initialization,
the transceiver is put into Pwon/Listen-Only mode for reading the PWON flag. If a LOW
signal is read on the pin ERRN, the ECU cold start was initiated by first battery power
application and the microcontroller performs the corresponding system start-up
procedure. If a HIGH signal is read, the cold start was initiated by a wakeup from Sleep
mode. In order to get information on the wakeup source, Normal mode is selected. If
reading pin ERRN yields a LOW signal, there was a local wakeup via the pin WAKE. If
reading yields a HIGH signal, the wakeup came via the bus. Afterwards, the cold start
procedure ends and normal operation continues.
13.3 Software flow for an ECU warm start
A warm start is performed when the ECU wakes up from Standby (low power level 1).
Figure 18 suggests a software flow for an ECU warm-start. The starting point assumes a
TJA1041 transceiver in its Standby mode and the host microcontroller in a dedicated
power-down mode if available. If the transceiver receives a wakeup either via the bus or
via the pin WAKE, the internal wakeup flag is set and signalled at the pin ERRN and RXD.
These signals can be used for wakeup of the microcontroller from its power-down mode.
The starting application program can now take control over the transceiver. If the PWON
flag is of interest, the microcontroller can force the transceiver into Pwon/Listen-Only
mode for reading the PWON flag. Otherwise the microcontroller can force the transceiver
directly into Normal mode for reading the Wakeup Source flag at the pin ERRN.
As the microcontroller remains powered by the VCC supply, the microcontroller can
monitor its port pins for possible wakeup events. On detection of a wakeup event the
microcontroller can initiate a wakeup by forcing the transceiver directly into Normal mode.
Then reading of the PWON flag or Wakeup Source flag is not necessary.
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e.g.
Transceiver in Standby
μC in Power-down
Vcc available
Warm Start
RXD-->0 and ERR-->0
Local or
Remote Wake-up
Wake-up Event
Select
Normal Mode
Set STB = 1
EN = 1
Wait 10us
Wake-up Source Flag
0 : Local Wake-up
1 : Wake-up via Bus
Read ERR
Activate
CAN-Controller
Release
CAN Reset
Set STB = 1
EN = 1
μC initiates
Wake-up
End of
Warm Start
Fig 18. Flow diagram for an ECU warm-start
13.4 How to enter Standby mode (low power level 1)
When the network management decides to put the bus system into Standby, each ECU
must receive an appropriate standby command. The flow diagram in Figure 19 shows the
different steps in order to put the TJA1041(A) into Standby mode.
On receiving a standby command (e.g. using a certain CAN message) the microcontroller
has to stop all CAN transmission. In order to ensure that no CAN communication is
present on the bus, caused by other nodes, the bus must have been recessive for a
suitable time before the TJA1041(A) is put into Standby mode by selecting STBN to LOW
and EN to LOW level. If there is no system dependent “waiting period” implemented there
would be the risk that a node sends out a last message while another one is already on
the way towards Standby mode. This would cause a wakeup event making it impossible to
enter a system wide low-power state.
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TJA1041 in
Normal Mode
Operating
Standby command
received
Stop all CAN
Transmission
Wait suitable time
for Bus "recessive"
Select
Standby Mode
Set STB = 0
EN = 0
Standby Mode
Fig 19. Flow diagram for entering standby mode
13.5 How to enter sleep mode (low power level 2)
The procedure to put an ECU into Sleep as shown in Figure 20 is similar to the previous
one for entering the Standby mode. On receiving a sleep command the microcontroller
has to stop all CAN transmission. To ensure that no CAN communication is present on the
bus, it must have been recessive for a suitable time before the TJA1041(A) is put into
Sleep mode by selecting STBN to LOW and EN to HIGH level. The difference now is that
the microcontroller checks periodically for a wakeup as long as VCC is not yet down. This
is necessary since it might happen that a wakeup event just appears while the Go-to
Sleep Command is processed. In this case the INH of the TJA1041(A) remains HIGH and
the VCC does not drop. Instead, the wakeup request is forwarded to the application via
RXD and ERR. Without this check the microcontroller would assume that a sleep phase
follows with disabled VCC, waiting forever for a power-on reset caused by a wakeup,
which never happens.
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TJA1041 in
Normal Mode
Operating
Sleep Command
received
Stop all CAN
Transmission
Wait suitable time
for Bus "recessive"
Select
Go-to-Sleep Mode
Check for
Wake-up
Set STB = 0
EN = 1
Read ERR/RXD
Yes
ERR/RXD=0 ?
No
Vcc not down
Vcc
down
Sleep Mode
Wake-up
Restart
Fig 20. Flow diagram for entering sleep mode
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14. Interoperability
Besides the TJA1041(A), NXP Semiconductors offers the PCA82C250, PCA82C251,
TJA1050 and TJA1040 high speed CAN transceiver products. Since all products are
compatible with the ISO11898 standard, interoperability with each other is in principle
guaranteed. They are able to work together in the same bus network. There are some
issues related to different bus biasing behavior during low-power operation, which shall be
considered in this chapter. Table 8 shows the bus biasing in the different operation modes
as well as in un-powered condition. Whenever there is a difference in the bus biasing, a
steady DC common mode current will flow within the system. The common mode input
resistance mainly defines the amount of this common mode current. This is shown in
Figure 21 for a bus in recessive state including TJA1041(A) and C250 nodes.
Table 8.
Bus biasing of NXP transceivers depending on operation mode
Transceiver
Operation mode
Bus bias
TJA1041
Normal, Pwon/Listen-Only
VCC/2
Standby, Sleep, Go-to-Sleep,
Un-powered
weak GND
Normal
VCC/2
Standby
weak GND
Un-powered
Floating
Normal, Silent
VCC/2
Un-powered
weak GND
Normal, Standby
VCC/2
Un-powered
GND
TJA1040
TJA1050
C250/C251
RCM,C250 /nC250
RCM,1041 /n1041
CANH
Icomp
Vcc/2
RCM,C250 /nC250
CANL
Powered
C250 nodes
RCM,1041 /n1041
TJA1041 nodes in
Sleep or Standby
Fig 21. Equivalent bus circuit for a mixed system of TJA1041(A) nodes in Standby/Sleep
mode and powered C250 nodes (in Standby or Normal mode)
Due to the big common mode input resistance CAN communication is not affected while
parts of the network are still in low-power mode, while other nodes have already started
communication. However, degradation of the emission performance is expected.
The following formula calculates the DC common mode current in a mixed system of
TJA1041(A) and C250 nodes.
V CC ⁄ 2
I comp,max = ------------------------------------------------------------------------------------------------------------------------R CM ( C250 ) ⁄ ( 2n C250 ) + R CM ( TJA1041 ) ⁄ ( 2n TJA1041 )
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with:
nC250: number of nodes powered C250
nTJA1041: number of nodes of TJA1041 in Standby/Sleep mode
RCM(C250) = 5k min common mode input resistance of C250 at pin CANH/L
RCM(TJA1041) = 15k min common mode input resistance of TJA1041 at pin CANH/L
Table 9.
Conditions leading to DC common mode current
Transceiver
TJA1040
TJA1050
C250/C251
TJA1041
Normal
PWON/Listen
only
Standby
Sleep
Go to sleep
Normal
---
---
X
X
X
Standby
X
X
---
---
---
Un-powered
---
---
---
---
---
Normal
---
---
X
X
X
Silent
---
---
X
X
X
Un-powered
X
X
---
---
---
Normal
---
---
X
X
X
Standby
---
---
X
X
X
Un-powered
X
X
---
---
---
X : DC common mode current --- : no DC common mode current
14.1 TJA1041(A) mixed with C250/C251/TJA1050 nodes
Table 9 identifies the conditions leading to different bus biasing and DC common mode
current. There is some compensation current in case TJA1041(A) nodes are in Normal
(High speed) mode, while other C250/C251/TJA1050 nodes are left un-powered.
Moreover, common mode current occurs when TJA1041(A) nodes are in
Standby/Sleep/Go-to-Sleep mode, while other C250/C251/TJA1050 nodes are kept
powered in any operation mode. However, the common mode current is negligible.
14.2 TJA1041(A) mixed with TJA1040 nodes
Table 9 reveals also that in a mixed system of TJA1040 and TJA1041(A) nodes, it is not
expected to have situations of different bus biasing. In the low-power modes both the
TJA1040 and TJA1041(A) show a weak termination to GND. When the bus is in
power-down with all nodes either in Standby or Sleep mode, there will be no DC common
mode current. During normal CAN operation, when all nodes are into Normal (High
speed) or Pwon/Listen-Only mode for diagnosis features, the bus is collectively biased to
VCC/2. There will be no DC common mode current.
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15. Appendix
15.1 Differences between TJA1041 and TJA1041(A)
The only difference between the TJA1041 and TJA1041A concerns the bus wakeup
mechanism. The TJA1041 detects a bus wake-up request when the bus remains in
dominant state for at least 5 µs. In contrast to that, the TJA1041A detects a bus wake-up
request when the bus shows two dominant phases of at least 5 µs duration, with each
dominant phase followed by a recessive phase of at least 5 µs. Figure 22 illustrates the
wake-up pattern requirements for the TJA1041 and TJA1041A.
The enhanced bus wakeup detection offers improved robustness against unwanted
wakeup in presence of bus failures, especially for large networks. There will be no
unwanted bus wake-up due to a BAT-to-CANH short-circuit or CANL wire interruption,
while the system is entering Bus Sleep.
At a data rate of 500 kbit/s, a single arbitrary CAN data frame is not necessarily sufficient
to launch a remote wake-up with the enhanced wake-up pattern. Here, two consecutive
arbitrary CAN data frames are needed to reliably launch a remote wake-up. At 125 kbit/s
data rate, any CAN data frame on the bus leads to a remote wake-up of the TJA1041A
transceiver.
dominant
bus wake-up pattern for
TJA1041
Vdiff,bus
> tBUS
dominant
recessive
dominant
recessive
bus wake-up pattern for
TJA1041A
Vdiff,bus
> tBUS
> tBUS
> tBUS
> tBUS
(tBUS : 0,75...5us)
Fig 22. Bus wake-up pattern for TJA1041 and TJA1041A
15.2 Maximum power dissipation within termination resistors
With the help of Figure 23 the maximum power dissipation within the termination resistors
in the case of a bus short circuit is calculated. Assuming a max. short-circuit voltage of
40 V on the bus, the max. short-circuit current, which may flow within the TJA1041 while
transmitting dominant, is limited to 95 mA Ref. 1. As shown in the figure, when one node
is transmitting a dominant bit, this short-circuit current is distributed to the two bus
terminations, so that each 60 Ω termination resistor would dissipate power of:
Pdom = 60 Ω x (95 mA / 2)2 = 135 mW
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The average power dissipation is reduced according to the average recessive/dominant
duty cycle on TXD.
VCC
VBAT
CANH
60Ω
60Ω
ISC(CANL)
2
ISC(CANL)
2
60Ω
CANL
60Ω
ISC(CANL)
Fig 23. Maximum power dissipation within termination resistors
15.3 Application with slow-starting VCC supply
For a successful start-up of the TJA1041(A) the VCC and VIO supply voltage has to ramp
up within at least the min. “under-voltage detection time on VCC or VIO“ Ref. 1 after BAT
power application or after a wakeup from Sleep mode (with disabled VCC supply). If the
INH controlled supply starts too slowly, the TJA1041(A) may detect a VCC/VIO
under-voltage condition, forcing the transceiver into Sleep mode. These applications with
slow starting supplies would need an extra local wakeup impulse via the pin WAKE to get
the node alive. Figure 24 sketches a possible solution, which can generate an extra local
wakeup impulse. It includes measures to elongate the INH “On” signal and a transistor
switch at the pin WAKE, such that a local wakeup pulse can be applied from the
microcontroller.
As the wakeup circuit needs some time for recovery (“under-voltage recover time on VCC
and VIO” Ref. 1) after a VCC/VIO under-voltage has been detected, the extra local wakeup
pulse should be given to the transceiver earliest after the max. recovery time from point of
the under-voltage detection. The diagram in Figure 25 illustrates the timing in more detail.
After BAT power application to the TJA1041(A) the transceiver is initialized, activating INH
almost immediately.
1. Because of the large delay in ramping up VCC, the transceiver may detect a VCC/VIO
under-voltage condition as early as 5 ms after battery power application, switching off
the INH pull-up current and leaving the pin floating.
2. However, due to the INH pulse lengthening, the voltage regulator still keeps active,
allowing complete ramp-up of VCC and initializing the microcontroller.
3. As soon as the maximum under-voltage recovery time Ref. 1 has expired, the
microcontroller can generate a local wakeup to the transceiver. This wakeup should
be applied first after VCC has completely ramped up. As a result the transceiver wakes
up from Sleep mode, entering the mode currently selected at the mode control pins
STBN and EN.
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pulse
lengthening
BAT
5V
VBAT VCC VIO
CANH
INH
SPLIT
TXD
CANL
TJA1041
RXD
uC
ERRN
20k
EN
WAKE
STBN
Px.y
3k
GND
Fig 24. Possible application with slow-starting VCC supply
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under-voltage
detected
1
Pin VBAT
min. 5ms
2
Pin Vcc
INH Pin
behaviour
Pull-up
floating
Pull-up
INH after pulse
lengthening
>50μs
Px.y
Under-voltage
recover time expired
3
uC generates a local
wakeup event
Pin WAKE
time
Fig 25. Wake-up timing with VCC/VIO undervoltage detection during start-up
15.4 VCC supply
Typically a capacitor between 47-100 nF is recommended being connected between VCC
and GND close to the transceiver. This capacitor buffers the supply voltage during the
transition from recessive to dominant, when there is a sharp rise in current demand. An
additional bypass capacitor is usually placed at the output of the voltage regulator. Its
purpose is to buffer disturbances on the battery line and to buffer extra supply current
demand in the case of bus failures. The calculation of the bypass capacitor value is shown
in Section 15.4.2 , while in Section 15.4.1 the average VCC supply current is calculated for
thermal load considerations of the VCC voltage regulator. This can be done in the absence
and in the presence of bus short-circuit conditions.
15.4.1 Thermal load consideration for the VCC voltage regulator
The average VCC supply current can be calculated in absence and in presence of bus
short-circuit conditions. The maximum average supply current in absence of bus failures
calculates to:
I CC_norm_avg = 0.5 ⋅ ( I CC_rec_max + I CC_dom_max )
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Example:
With ICC_rec_max = 10 mA and ICC_dom_max = 80 mA Ref. 1 this results in an average supply
current of 45 mA. Here a transmit duty cycle of 50 % on pin TXD is assumed.
In presence of bus failures the VCC supply current for the transceiver can increase
significantly. The maximum dominant VCC supply current ICC_dom_sc_max flows in the case
of a short circuit from CANH to GND. Along with the CANH short circuit output current
ISC(CANH) the maximum dominant VCC supply current ICC_dom_sc_max calculates to about
120 mA. This results in an average supply current of 65 mA in worst case of a short circuit
from CANH to GND. The VCC voltage regulator must be able to handle this average
supply current.
15.4.2 Dimensioning the bypass capacitor of the voltage regulator
Dimensioning the bypass capacitor gets very important with a shared voltage supply
between transceiver and microcontroller. Here, extra current demand with bus failures
may not lead to an unstable supply for the microcontroller. Following the extra current
demand in the case of bus failures is calculated. This input is used to determine the
bypass capacitor needed to keep the voltage supply stable under the assumption that all
the extra current demand has to be delivered from the bypass capacitor.
The quiescent current delivered from the voltage regulator to the transceiver is
determined by the recessive VCC supply current ICC_rec.
Extra supply current is demanded during dominant transmitting periods. It is calculated by:
ΔI CC_ex = I CC_dom – I CC_rec
(5)
In absence of bus failures the maximum extra supply current is calculated by:
ΔI CC_max = I CC_dom_max – I CC_rec_min
(6)
Example:
With ICC_dom_max = 80 mA and ICC_rec_min = 2 mA the maximum extra supply current
calculates to 78 mA. Normally, the voltage regulator is strong enough to deliver this extra
supply current without significant VCC voltage drop.
In presence of bus failures the maximum extra supply current may be significantly higher.
Considering the worst case of a short circuit from CANH to GND the maximum extra
supply current is calculated by:
ΔI CC_max_sc = I CC_dom_sc_max – I CC_rec_sc_min
(7)
Example:
With ICC_dom_sc_max = 120 mA (estimated) and ICC_rec_sc_min = 2 mA the maximum extra
supply current calculates to 118 mA.
In the case of a short circuit from CANH to GND, the bus is clamped to the recessive
state, and according to the CAN protocol the CC transmits 17 subsequent dominant bits
on TXD. That would mean the above calculated maximum extra supply current has to be
delivered for at least 17 bit times. The reason for the 17 bit times is that at the moment the
CAN controller starts a transmission, the dominant Start Of Frame bit is not fed back to
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RXD and forces an error frame due to the bit failure condition. The first bit of the error
frame again is not reflected at RXD and forces the next error frame (TX Error Counter +8).
Latest after 17 bit times, depending on the TX Error Counter Level before starting this
transmission, the CAN controller reaches the Error Passive limit (128) and stops sending
dominant bits. Now a sequence of 25 recessive bits follows (8 Bit Error Delimiter + 3 Bit
Intermission + 8 Bit Suspend Transmission) and the VCC supply current becomes reduced
to the recessive one.
Assuming that the complete extra supply current during the 17 bit times has to be buffered
by the bypass capacitor, the worst-case bypass capacitor calculates to:
ΔI CC_max_sc ⋅ t dom_max
118 mA ⋅ 34 μs
C Buff = --------------------------------------------------- = -------------------------------------- ≈ 20 μF
0.2 V
ΔV max
(8)
with:
ΔI CC_max_sc = 118 mA
(9)
maximum extra supply current in the case of a short circuit from CANH to GND:
t dom_max = 34 μs
(10)
dominant time of 17 bit times at 500 kbit/s:
ΔV max = 0.2 V
(11)
maximum allowed VCC voltage drop:
Of course, depending on the regulation capabilities of the used voltage regulator the
bypass capacitor may be much smaller.
16. Abbreviations
Table 10.
Acronym
Abbreviations
Description
CAN
Controller Area Network
Clamp-15
ECU architecture, Battery supply line after the ignition key, module is
temporarily supplied by the battery only (when ignition key is on)
Clamp-30
ECU architecture, direct battery supply line before the ignition key, module is
permanently supplied by the battery
ECU
Electronic Control Unit
EMC
Electromagnetic Compatibility
EME
Electromagnetic Emission
EMI
Electromagnetic Immunity
FMEA
Failure Mode and Effects Analysis
SOI
Silicon On Insulator
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17. References
[1]
Data Sheet TJA1041,TJA1041A High-Speed CAN transceiver — Philips
Semiconductors, 2003 Oct 14
[2]
Road Vehicles - Interchange of Digital Information - Controller Area Network
(CAN) for high-speed communication — ISO11898, International Standardization
Organisation, 1993
(Under review at ISO TC22/SC3/WG1)
[3]
High Speed CAN (HSC) for Vehicle Applications at 500kbps — SAE J2284,
1999
[4]
Data Sheet TJA1054, Fault-tolerant CAN transceiver — Philips Semiconductors,
2001 Nov 20
[5]
Data Sheet PCA82C250, CAN controller interface — Philips Semiconductors,
2000 Jan 13
[6]
Data Sheet PCA82C251, CAN controller interface — Philips Semiconductors,
2000 Jan 13
[7]
Data Sheet TJA1050, High speed CAN transceiver — Philips Semiconductors,
2003 Oct 22
[8]
Data Sheet TJA1040, High speed CAN transceiver — Philips Semiconductors,
2003 Oct 14
[9]
SAE Conference Paper 950298, EMC Measures for Class C Communication
Systems using Unshielded Cable — Lütjens/Eisele, Detroit 1995
[10] IDB-C Physical Layer, Draft Recommended Practice — SAE J2366-1, 2001 May
04
[11] Road Vehicles – Diagnostics on CAN - Part 1,2,3,4 — ISO/WD 15765,
International Standardization Organisation, 1999 Nov 30
[12] Application Note AN00020, TJA1050 CAN High-Speed Transceiver — NXP
Semiconductors, 2006 Nov 08
[13] Objective Data Sheet PESD1CAN, CAN bus ESD protection diode — Philips
Semiconductors, 2005 Oct 17
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18. Legal information
18.1 Definitions
Draft — The document is a draft version only. The content is still under
internal review and subject to formal approval, which may result in
modifications or additions. NXP Semiconductors does not give any
representations or warranties as to the accuracy or completeness of
information included herein and shall have no liability for the consequences of
use of such information.
Suitability for use — NXP Semiconductors products are not designed,
authorized or warranted to be suitable for use in medical, military, aircraft,
space or life support equipment, nor in applications where failure or
malfunction of a NXP Semiconductors product can reasonably be expected to
result in personal injury, death or severe property or environmental damage.
NXP Semiconductors accepts no liability for inclusion and/or use of NXP
Semiconductors products in such equipment or applications and therefore
such inclusion and/or use is at the customer’s own risk.
Applications — Applications that are described herein for any of these
products are for illustrative purposes only. NXP Semiconductors makes no
representation or warranty that such applications will be suitable for the
specified use without further testing or modification.
18.2 Disclaimers
General — Information in this document is believed to be accurate and
reliable. However, NXP Semiconductors does not give any representations or
warranties, expressed or implied, as to the accuracy or completeness of such
information and shall have no liability for the consequences of use of such
information.
18.3 Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
Right to make changes — NXP Semiconductors reserves the right to make
changes to information published in this document, including without
limitation specifications and product descriptions, at any time and without
notice. This document supersedes and replaces all information supplied prior
to the publication hereof.
AN00094_3
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19. Contents
1
2
3
4
4.1
4.2
4.3
4.4
4.5
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
General application of high speed CAN . . . . . 3
Target applications for TJA1041/TJA1041A . . 7
Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Low power management . . . . . . . . . . . . . . . . . 8
Bus failure diagnosis . . . . . . . . . . . . . . . . . . . 11
System fail-safe features . . . . . . . . . . . . . . . . 11
Common mode stabilization . . . . . . . . . . . . . . 11
I/O level adaptation to host controller supply
voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5
Operating modes . . . . . . . . . . . . . . . . . . . . . . . 11
5.1
Normal mode . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2
Pwon/Listen-only mode . . . . . . . . . . . . . . . . . 12
5.3
Standby mode. . . . . . . . . . . . . . . . . . . . . . . . . 12
5.4
Sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.5
Go-to Sleep Command mode . . . . . . . . . . . . . 14
6
Hardware application of the TJA1041(A) . . . . 15
6.1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1
Pin VBAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.2
Pin VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.3
Pin VIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.4
Pin TXD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.5
Pin RXD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.6
Pin STBN/EN . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.7
Pin ERRN . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.8
Pin CANH/L . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.9
Pin SPLIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.10
Pin WAKE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.11
Pin INH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2
Application of common mode choke. . . . . . . . 18
6.3
ESD protection . . . . . . . . . . . . . . . . . . . . . . . . 19
6.4
PCB layout rules . . . . . . . . . . . . . . . . . . . . . . . 19
7
Wakeup detection . . . . . . . . . . . . . . . . . . . . . . 19
7.1
Wakeup via bus . . . . . . . . . . . . . . . . . . . . . . . 19
7.2
Wakeup via WAKE pin . . . . . . . . . . . . . . . . . . 20
7.2.1
Dimensioning of RS and Rexbias . . . . . . . . . . . 21
8
Flag signaling. . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1
Wakeup flag . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.2
PWON flag . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.3
Wakeup source flag . . . . . . . . . . . . . . . . . . . . 23
8.4
Bus failure flag . . . . . . . . . . . . . . . . . . . . . . . . 23
8.5
Local failure flag . . . . . . . . . . . . . . . . . . . . . . . 24
9
Bus failure diagnosis . . . . . . . . . . . . . . . . . . . 25
9.1
List of signaled bus failures . . . . . . . . . . . . . . 25
9.2
How to read the bus failure flag . . . . . . . . . . . 25
10
Local failure diagnosis . . . . . . . . . . . . . . . . . . 27
10.1
10.2
10.3
10.4
10.5
10.6
11
11.1
11.2
12
13
13.1
Recovery from local failures. . . . . . . . . . . . . . 27
TXD dominant clamping. . . . . . . . . . . . . . . . . 27
TXD/RXD short circuit . . . . . . . . . . . . . . . . . . 27
RXD recessive clamping . . . . . . . . . . . . . . . . 28
Bus dominant clamping . . . . . . . . . . . . . . . . . 28
Over temperature protection . . . . . . . . . . . . . 29
Under-voltage detection . . . . . . . . . . . . . . . . . 30
VCC/VIO under-voltage detection . . . . . . . . . . 30
VBAT under-voltage detection . . . . . . . . . . . . . 30
Pin FMEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Software issues . . . . . . . . . . . . . . . . . . . . . . . . 33
Software flow for handling communication
failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.2
Software flow for an ECU cold start . . . . . . . . 35
13.3
Software flow for an ECU warm start . . . . . . . 36
13.4
How to enter Standby mode (low power
level 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.5
How to enter sleep mode (low power level 2) 38
14
Interoperability . . . . . . . . . . . . . . . . . . . . . . . . 40
14.1
TJA1041(A) mixed with C250/C251/TJA1050
nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
14.2
TJA1041(A) mixed with TJA1040 nodes . . . . 41
15
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
15.1
Differences between TJA1041 and
TJA1041(A) . . . . . . . . . . . . . . . . . . . . . . . . . . 42
15.2
Maximum power dissipation within termination
resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
15.3
Application with slow-starting VCC supply . . . 43
15.4
VCC supply . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
15.4.1
Thermal load consideration for the VCC voltage
regulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
15.4.2
Dimensioning the bypass capacitor of the voltage
regulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
16
Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . 47
17
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
18
Legal information . . . . . . . . . . . . . . . . . . . . . . 49
18.1
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
18.2
Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . 49
18.3
Trademarks . . . . . . . . . . . . . . . . . . . . . . . . . . 49
19
Contents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
© NXP B.V. 2006.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: [email protected]
Date of release: 8 November 2006
Document identifier: AN00094_3