AN1365

AN1365
Recommended Usage of Microchip Serial RTCC Devices
Author:
Martin Bowman
Microchip Technology Inc.
INTRODUCTION
Many embedded systems require some form of accurate timekeeping. There are a growing number of applications that require an external Real-Time Clock/
Calendar (RTCC) and higher integration of external
peripheral components into the RTCC. In order to
achieve a highly robust and repeatable system, the
designer must consider the rest of the system components including pull-up resistor values and the crystal
selection. There are a number of situations that can
result in less than optimal operation, many of which are
easy mistakes that are avoidable with some initial
knowledge. These are discussed in this application
note.
This application note provides assistance and
guidance in using the Microchip RTCC family of
devices. This application note covers both the I2C™
(MCP794XX) and SPI (MCP795XXX) family of
devices. These recommendations are not meant as
requirements, however, their adoption will lead to a
more robust overall design. The following topics are
discussed:
•
•
•
•
•
•
•
Basic Design Considerations
VCC Supply
Backup Supply options
Input Pins
Output Pins
Crystal Selection
Recommended Schematics (Appendix B-D)
All of the recommended practices that are detailed in
this document are used on the RTCC PICtail™
daughter boards available from Microchip.
Appendix B: “Recommended Connections for
MCP794XX Series Devices” shows the suggested
connections for using the Microchip I2C MCP794XX
RTCC family.
Appendix C: “Recommended Connections for
MCP795XX Series Devices” and Appendix D: “Recommended Connections for MCP795WXX Series
Devices” show similar schematic for the SPI RTCC
devices. The basis for these connections will be
explained in the following sections.
 2010-2013 Microchip Technology Inc.
POWER SUPPLY
Microchip I2C RTCC devices feature a robust serial
communication protocol that guards against unintentional writes and data corruption while power is within
normal operating levels.
The Microchip Serial RTCC devices operate over a
wide voltage range. Two power supplies are required
for full device operation:
• Main VCC – Required for full Read/Write functionality and clock/calendar operation.
• VBAT Supply – Required to maintain the clock/
calendar during the time when VCC is not present.
Please refer to the device data sheet for voltage range
information. Additional information regarding the VBAT
supply is provided later in this text.
As is shown in the schematics in Appendix B through
D, a decoupling capacitor (typically 0.1 F) should be
used to help filter out noise on VCC.
Power-Up
On power-up, VCC should always begin at 0V and rise
to its normal operating voltage to ensure a proper
Power-on Reset. VCC should not linger at an
ambiguous voltage (i.e., below the minimum operating
voltage).
However, if VCC happens to fall below the minimum
retention voltage for the device (see data sheet DC
Characteristics), it is recommended that VCC be
brought down fully to 0V before returning to normal
operating level. This will help to ensure that the device
is reset properly.
Furthermore, if the microcontroller features a Brownout Reset with a threshold higher than that of the
RTCC, bringing VCC down to 0V will allow both devices
to be reset together. Otherwise, the microcontroller
may reset during communication while the RTCC is still
in an operational condition.
VCC Ramp Rates
The Microchip RTCC family integrates a battery switch
over circuit to maintain the time and also the contents
of the SRAM during the time when VCC is below the
VTRIP threshold as defined in the data sheet. Due to the
fact that the circuit operates at a very low current level,
care should be exercised to ensure that the rise and fall
times listed in the data sheet are met.
DS00001365D-page 1
MCP794XX/MCP795XXX
Many applications will meet these requirements simply
based on the capacitance on the VCC lines and also the
output impedance of the power supply circuit and the
PCB copper resistance.
FIGURE 1:
The following data sheet timing specifications should
be met.
• TFvcc – VTRIP(max) to VTRIP(min)
• TRvcc – VTRIP(min) to VTRIP(max)
VTRIP GRAPH
VCC
VTRIP(max)
VTRIP(min)
Internal Switch to VBAT
Internally, the RTCC will switch to the VBAT supply
when VCC drops to the VTRIP voltage detailed in the
data sheet.
Failure of VCC During a Read
During a read of the RTCC registers, SRAM or
EEPROM, if the VCC supply drops, the device will continue to operate as per the device data sheet and
communication is still possible with the device until VCC
reaches the VBAT trip point.
Failure of VCC During an EEPROM Write
During the time that data is being written to the
EEPROM or unique ID locations, VCC should remain
above the minimum operating voltage – typically 1.8V.
If at any time VDD drops below this minimum voltage
but remains above the VBAT switch over voltage (VTRIP
as specified in the device data sheet) then care should
be taken to ensure that the data written to the device is
free from errors by verifying the contents of the memory
written.
If at any time the VCC voltage drops below 1.5V (VBAT
switch over) then the I2C and SPI interface is disabled
and any writes that are in process will be terminated. It
is recommended that, after such a condition, the
EEPROM locations that were being written are verified.
DS00001365D-page 2
TRVCC
TFVCC
Time-stamp
(power loss)
Time-stamp
(power restore)
Failure of VCC During an SRAM or RTCC
Write
SRAM and RTCC writes are possible when VCC is
dropping until the VBAT trip point is reached. It is not
recommended to communicate during this time and all
I2C and SPI communication should be stopped as soon
as possible if the system is able to detect a power-fail
condition.
VBAT Selection
This is not applicable to the MCP7940M device.
The external VBAT pin supplies power to maintain the
RTCC and also the SRAM during a VCC power fail. If
this function is not required, then the VBAT pin should
be connected to GND. Connecting this pin to GND will
result in the lowest current configuration.
The supported voltage on this pin is from 1.3V to 5.5V.
The internal circuit will switch to the VBAT voltage when
VCC drops to 1.5V (data sheet parameter VTRIP). The
RTCC and SRAM will continue to be maintained until
the VBAT voltage drops to 1.3V.
The Microchip RTCC devices will support both primary
backup supplies (battery etc.) and also rechargeable
solutions (NiCad, Super Cap, etc). When using any
supply it is recommended to include a 1K series resistor between the supply and the VBAT pin and a 100pF
capacitor between VBAT pin and GND. Additionally, a
series diode is recommended when using a nonrechargeable supply to eliminate any current flowing
into the cell during a device failure.
 2010-2013 Microchip Technology Inc.
MCP794XX/MCP795XXX
When using a rechargeable solution, additional components will be required to support a charge current to
maintain the voltage on the battery/capacitor. Care
should be exercised to ensure that the backup supply
cannot power the VCC supply during a main supply failure, this is accomplished using a diode in series with
the current limit resistor. Figure 2 shows a typical schematic for using a supercap, the same schematic would
also apply to a rechargable battery.
FIGURE 2:
SUPERCAP
VCC
EVHS and EVLS
These pins are only available on the following device:
• MCP795WXX – 14-pin SPI RTCC Family
The High-Speed Event (EVHS) detect and Low-Speed
Event (EVLS) detect are digital input pins and require
either a pull-up or pull-down resistor.
These pins are used as the input to the Event Detection
circuit. If this feature is not being used in the application
then these inputs should be connected to GND.
SERIAL COMMUNICATION PINS
SPI Communication
Shottky Diode
Sized to limit
charge current
and charge
time
The MCP795XXX supports the industry standard SPI
bus protocol using the SCK, SD, SO and CS Lines.
1K Ohms
Backup
Capacitor
VBAT
100pF
RTCC
The CS line must be brought low at the start of a command and raised at the end of the command. The CS
line being raised completes the command and performs the write cycle for a nonvolatile memory write.
The CS line should not toggle during the command
sequence, as raising the CS line before the command
is complete terminates the current command. A pull up
is recommended on CS to ensure that the RTCC powers up in an unselected state.
I2C Communication
UL Considerations
One of the requirements for UL approval and certification is related to the VBAT supply. If a lithium primary
cell is used (CR2032 or similar), then there are reverse
leakage currents that have to be taken into consideration. By using the recommended low voltage drop
(Schottky) diode in series with the lithium backup battery, this issue is limited.
In addition to the recommended diode and series resistor, internally the VBAT/VCC switch over circuit has been
designed such that in the event of a catastrophic failure
of the device, the switch will fail in a safe manner and
not conduct from VCC to VBAT.
INPUT-ONLY PINS
It is never good practice to leave a digital input pin floating. This can cause an elevated standby current as well
as undesired functionality. If a pin is left floating, it can
float either low or high. The final logic state is dependent upon a number of factors, including noise in the
system and capacitive coupling. Because of this, the
level seen by the input circuitry is relatively random and
likely to change during operation. This applies to the
EVHS and EVLS pins detailed below.
 2010-2013 Microchip Technology Inc.
The MCP794XX supports an I2C-compatible serial
interface. To follow the I2C specification, both the Serial
Data (SDA) and Serial Clock (SCL) lines require a pull
up to VCC. As the MCP794XX is designed to run at a
maximum of 400 kHz, suggested values at this speed
for both SCL and SDA are 2.2K Ohms at 5.5V.
Application Note AN1028, “Recommended Usage of
Microchip I2C™ Serial EEPROM Devices”, on the
Microchip web site, provides additional guidance for
the use and implementation of the I2C bus.
OUTPUT-ONLY PINS
MFP Pin
This Pin is available on the following devices:
• MCP794XX – I2C RTCC Family
• MCP795XX – 10-pin SPI RTCC Family
The multi-function pin (MFP) is used for a number of
functions when enabled by the RTCC registers. As this
pin is an open-drain output, a pull up is required to VCC
(it is not recommended to use a pull up to the VBAT
timekeeping supply).
This pin can sink a maximum of 10mA.
DS00001365D-page 3
MCP794XX/MCP795XXX
FIGURE 3:
MFP DIAGRAM
V CC
The WDO and IRQ pins are open-drain and are capable of sinking 10mA (Please refer to the DC Characteristics in the data sheet). A pull-up to VCC is required on
these pins.
The WDO and IRQ pins are used as the output from the
on-board watchdog timer and the alarm interrupt event.
RTCC
D evice
M FP
P IN
If the WDO and IRQ pins are not used they can be left
floating.
CRYSTAL SELECTION
The MCP794XX and MCP795XXX have been
designed to operate with a standard 32.768 kHz tuning
fork crystal with external loading capacitors.
Suitable crystals have a load capacitance of 6-9pF. It is
not recommended using crystals with a load capacitance of 12.5pF.
The MFP pin is used for the following operation when
VCC is present on the device:
• Alarm output – an active alarm generated from
one of the programmable alarms will assert this
line (pull the line low). The line can be wire OR’d
to other open-drain signals to drive a single MCU
IRQ line.
• General purpose output – can be used as an
additional I/O line under the control of the MCU.
• Output a clock signal – can be used to output a
frequency derived from the 32.768 kHz crystal. As
this is an open drain, the size of the pull-up resistor and the bus capacitance of that line will determine the rise and fall time of the signal.
For a list of tested crystals and suggested load capacitors, please refer to AN1519, “Recommended Crystals
for Microchip Stand-Alone Real-Time Clock/Calendar
Devices”.
One of the key points in selecting a crystal and load
capacitors is the load capacitance of the crystal. A crystal with a specified CL of 7pF will not operate at the
desired frequency using two 7pF capacitors. The CL is
the effective load capacitance, which includes the
physical capacitors, pin capacitance and stray board
capacitance. When calculating the effective load
capacitance, Equation 1 can be used:
EQUATION 1:
When VCC is removed and the device is running from
the backup supply, VBAT, the only functions that are
active on this pin are the alarms; all other functions are
disabled until VCC is restored.
CLKOUT Pin
Cx2  C x1
CL = ------------------------- + C stray
C x2 + C x1
FIGURE 4:
OSCILLATOR DIAGRAM
This pin is only available on the following device:
CX1
• MCP795WXX – 14-Pin SPI RTCC Family
The CLKOUT is a push/pull output that can produce a
square-wave that is derived from the crystal and onboard oscillator.
Please consult the device data sheet for the source/
sink specifications of this pin. If this pin is used to provide a clock source to another device, care must be
taken to ensure that the load of the driven device does
not exceed the drive capability of this pin.
If this pin is not used it can be left floating; do not connect to VCC or GND, as this is a digital output.
WDO and IRQ
X1
RTCC
X2
CX2
The recommended board layout for the oscillator area
for the MCP794XX (also applicable to the
MCP795XXX) is shown in Figure 4.
These pins are only available on the following device:
• MCP795WXX – 14-pin SPI RTCC Family
DS00001365D-page 4
 2010-2013 Microchip Technology Inc.
MCP794XX/MCP795XXX
Oscillator Layout
SUMMARY
Given that the oscillator is designed for minimum
operating current, care must be taken when laying out
the PCB traces. This is discussed below.
This application note illustrates recommended
techniques for increasing design robustness when
using the Microchip family of RTCC’s. These recommendations fall directly in line with how Microchip
designs, manufactures, qualifies and tests its RTCC
devices and will allow the devices to operate within the
data sheet parameters. It also serves to explain in
detail some of the features of the device and makes the
user aware of any potential pitfalls that may be encountered.
• Keep traces as short as possible to the crystal
and the load capacitors. Minimizing the length is
important to keep stray capacitance to a minimum. For that reason, it is not recommended to
use any kind of a socket, or package interposer
when developing with the RTCC devices. An
alternative that can be used is the RTCC PICtail
daughter board.
• Use a ground ring. During the PCB layout, a
ground ring should be placed around both the
crystal and also the X1 and X2 pins (pins 1, 2) on
the device. This ground ring should be connected
to a low-impedance ground connection. A
recommended layout is shown in Figure 5. In the
PCB layout example below, C2 and C3 are the
load capacitors CX1 and CX2.
FIGURE 5:
This document should be read in conjunction with the
following additional resources:
• Device Data Sheet
• AN1491, “Configuring the MCP794XX RTCC
Family”
• AN1496, “Debugging Stand-Alone Real-Time
Clock/Calendar-Based Applications”
• AN1519, “Recommended Crystals for Microchip
Stand-Alone Real-Time Clock/Calendar Devices”
CRYSTAL LAYOUT
(MCP794XX SHOWN)
It is recommended that the final application be tested
with the chosen crystal and capacitor across all environmental and operating conditions.
The Gerber files for the PICtail daughter board are
available on the web site following the link on
www.microchip.com/rtcc.
 2010-2013 Microchip Technology Inc.
DS00001365D-page 5
MCP794XX/MCP795XXX
APPENDIX A:
REVISION HISTORY
Revision C (11/2011)
Changed part number from MCP795XX
MCP795XXX; Added Revision History.
to
Revision D (03/2013)
Document format changed. Added additional information for crystal selection. Removed incorrect
references.
DS00001365D-page 6
 2010-2013 Microchip Technology Inc.
MCP794XX/MCP795XXX
VCC
10K
VCC
2K
VCC
RECOMMENDED CONNECTIONS FOR MCP794XX SERIES DEVICES
2K
APPENDIX B:
6
5
MCU
8
X1
SCL
SDA
X2
MCP794XX
7
VBAT
CX1
1
32.768
kHz
2
CX2
3
1K
Diode
MFP
4
 2010-2013 Microchip Technology Inc.
.1µF
100pF
DS00001365D-page 7
MCP794XX/MCP795XXX
VCC
10K
VCC
RECOMMENDED CONNECTIONS FOR MCP795XX SERIES DEVICES
MCU
VCC
10K
APPENDIX C:
4
8
7
6
9
.1µF
10
X1
CS
SCK
SO
SI
X2
VBAT
CX1
1
32.768
kHz
2
CX2
3
1K
Diode
MFP
5
100pF
MCP795XX
DS00001365D-page 8
 2010-2013 Microchip Technology Inc.
MCP794XX/MCP795XXX
APPENDIX D:
VCC
RECOMMENDED CONNECTIONS FOR MCP795WXX SERIES DEVICES
VCC
VCC
10K x 3
.1µF
14
MCU
6
10
8
9
X1
CS
SCK
SO
SI
X2
CX1
1
32.768
kHz
2
CX2
EVHS 12
13
CLKOUT
EVLS 11
4
WDO VBAT 3
5
IRQ
7
1K
Diode
100pF
MCP795WXX
Note: Both the EVHS and EVLS should be externally connected to either pull-up or pull-down resistors depending
on the polarity of the trigger.
 2010-2013 Microchip Technology Inc.
DS00001365D-page 9
MCP794XX/MCP795XXX
NOTES:
DS00001365D-page 10
 2010-2013 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
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ISBN: 9781620771280
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DS00001365D-page 11
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DS00001365D-page 12
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