Using the SENT Communications Output Protocol with A1341 and A1343 Devices

Product Information
Using the SENT Communications Output Protocol
with A1341 and A1343 Devices
By Nevenka Kozomora
Allegro MicroSystems, LLC
System Requirements
Allegro™ MicroSystems supports the Single-Edge Nibble
Transmission (SENT) protocol in certain advanced digital
output sensor ICs. The SENT protocol is a commonly
accepted automotive protocol for highly efficient transfer
of sensor data along intravehicular communications
networks, and is standardized by the Society of Automotive
Engineering in publication SAEJ2716.
The Allegro devices comply with the SENT 3-wire standard:
providing power along the 5 V wire, a logic-level signal output, and a ground reference. Specific devices may provide
additional capabilities with other pin configurations.
The system host controller must be capable of handling
at least 20 bits of data, including data, cyclic redundancy
checking (CRC), system status, and communication status.
This application note provides a description of the Allegro
implementation of the SENT protocol, which includes
extensions developed by Allegro to enhance the information
carrying dimensions of the output from the Allegro sensor
IC to the vehicle electronic control units (ECU).
Table of Contents
System Requirements
SENT Protocol Overview
SENT Output Mode
Message Structure
Data Nibble Format
Output Message Transfer
Optional Serial Output Protocol
Device Response Time
Propagation Delay and Output Update Rate
Asynchronous Transfer Minimum Response Time Asynchronous Transfer Maximum Response Time 296108-AN
1
2
3
4
6
7
12
13
13
14
15
Asynchronous Transfer with SENT Messages of Equal
Duration Maximum Response Time
Synchronous Transfer Mode Minimum Response Time Synchronous Transfer Maximum Response Time Device Response Time with Continuous Field Change
Fast SENT Feature
Minimum Message Length
Device Response Time Example Calculation
Electrical Specifications
SENT Data Programming Parameters
16
17
18
19
20
20
22
26
27
SENT Protocol Overview
The Allegro implementation of the SENT protocol complies with
the J2716 Rev. 2010 SENT standard. The Allegro sensor IC takes
the role of Slave in the SENT serial communications. In this role,
the Allegro device sends information about the magnetic field
applied to the device and about the internal status of the device.
The Allegro device sends both types of information from the
device output pin.
The Allegro implementation of the SENT protocol has various
programmable options:
• Clock Rates from 0.25 to 31.75 µs
• Type and quantity of Data nibbles
• Output Frame Rate
• Duration of nibble low state
• Polarity on SENT output (to invert the signal)
Two communications states are supported (figure 1):
• Status and Communication nibble format (error and serial
protocol)
• Default state: Slave sends messages to Master continuously.
• Programmable State: Slave sends one message to the Master
after receiving a trigger signal from the Master.
• Adjustable SENT nibble fall time
The Allegro implementation of the SENT protocol enables the
user to speed-up communication by using minimum tick time,
minimum fixed time in the nibble, and minimum quantity of
SENT nibbles in a message.
Continuous Transfer Mode
Master
(System
Microcontroller)
Continuous
Messages
Slave
(Allegro
Sensor IC)
Optional Triggered Transfer Mode
Master
(System
Microcontroller)
Trigger
Message
Slave
(Allegro
Sensor IC)
Figure 1. Message communication from the Allegro IC can be either:
continuous (upper panel) or individual messages can be in response to a
trigger signal from the Master (lower panel).
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SENT Output Mode
The SENT output mode converts the input magnetic signal to a
binary value digitally preprocessed and mapped to a Full-Scale
Output (FSO) range as shown in figure 2. This data is inserted
into a binary pulse message, referred to as a frame, that conforms to the SENT data transmission specification (SAEJ2716
JAN2010).
Certain parameters for configuration of the SENT messages can
be set in EEPROM.
The SENT output mode is configured by setting the following
parameters in EEPROM:
• PWM_MODE parameter set to 0 (default) to select the SENT
option
• SENT_x programming parameters (see EEPROM Structure
section)
Magnetic Signal,
BIN (G)
4095 (1111 1111 1111 1111)
2048 (1000 0000 0000 0000)
0000 (0000 0000 0000 0000)
SENT Data Value
(LSB)
Nibble fall time is changed by changing the drive current to the
output pin.
Figure 2. SENT mode outputs a digital value that can be read by the external controller.
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Message Structure
A SENT message is a series of nibbles, with the following characteristics:
val (the SENT_LOVAR parameter selects the interval, and
SENT_FIXED sets the duration).
• The other interval in the pair, high-voltage, becomes the information state and is variable in duration, depending on the nibble
data value. See table 1.
• Each nibble is a pair of voltage intervals: a low-voltage interval
and a high-voltage interval (figure 3).
• The time duration of the nibble depends on the total duration,
determined by the total quantity of time units, referred to as
ticks, and the information contained by the nibble. The duration
of a tick is set by dividing a 4 MHz clock by the value of the
SENT_TICK parameter. The duration of the nibble is the sum
of the low-voltage interval plus the high-voltage interval.
The nibbles of a SENT message are arranged in the following
required sequence (see figure 4 and table 2):
1. Synchronization and Calibration: flags the start of the SENT
message
2. Status and Communication: provides the device status and the
format of the data
3. Data: magnetic field and optional data
4. CRC: error checking
5. Pause Pulse (optional): sets timing relative to device updates
• The low-voltage interval is by default the delimiting state,
which only sets a boundary for the nibble; to assign the delimiting state, select a fixed number of ticks for the inter-
0
5
12
Ticks
0
5
Table 1. Nibble Composition and Value
27
Quantity of Ticks per Nibble
Ticks
Message
Signal
Voltage
Message
Signal
Voltage
Low
High
Interval Interval
Low
Interval
Nibble Data Value = 0000
High
Interval
Nibble Data Value = 1111
12
0000
0
8
13
0001
1
9
14
0002
2
5
5
…
…
…
SENT_FIXED
7
5
…
SENT_FIXED
Decimal
Equivalent
Value
HighVoltage
Interval
…
Figure 3. General value formulation for SENT nibble: (left) 0000,
(right) 1111 (see table 1 for correspondence)
Total
Binary
(4-Bit)
Value
LowVoltage
Interval
5
21
26
1110
14
5
22
27
1111
15
SENT_FIXED
SENT_FIXED
SENT_FIXED
SENT_FIXED
SENT_LOVAR = 0
12 to 27
ticks
56 ticks
Nibble Name
Synchronization
and Calibration
56 ticks
SENT_LOVAR = 1
12 to 27
ticks
12 to 27
ticks
12 to 27
ticks
Data 1
(MSB)
Data n
CRC
12 to 27
ticks
12 to 27
ticks
12 to 27
ticks
Status and
Communication
12 to 27
ticks
SENT_FIXED
SENT_FIXED
SENT_FIXED
SENT_FIXED
Pause
Pulse
(optional)
SENT_FIXED
SENT_FIXED
tSENT
Figure 4. General format for SENT message frame: (upper panel) low state fixed, (lower panel) high state fixed
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Table 2. SENT Message Frame Section Definitions
Section
Description
Synchronization and Calibration
Function
Provide the external controller with a detectable start of the message frame. The large
quantity of ticks distinguishes this section, for ease of distinction by the external controller.
Nibbles: 1
Syntax Quantity of ticks: 56
Quantity of bits: 1
Status and Communication
Function
Provides the external controller with the status of the device and indicates the format and
contents of the Data section.
Nibbles: 1
Quantity of ticks: 12 to 27
Syntax Quantity of bits: 4
1:0 Device status (set by SENT_STATUS parameter)
3:2 Message serial data protocol (set by SENT_SERIAL parameter)
Data
Function Provides the external controller with data selected by the SENT_SERIAL parameter.
Nibbles: 3 to 6
Syntax Quantity of ticks: 12 to 27 (each nibble)
Quantity of bits: 4 (each nibble)
CRC
Function
Syntax
Provides the external controller with cyclic redundancy check (CRC) data for certain error
detection routines applied to the Data nibbles and to the Status information.
Nibbles: 1
Quantity of ticks: 12 to 27 (each nibble)
Quantity of bits: 4
Pause Pulse
Function
(Optional) Additional time can be added at the end of a SENT message frame to ensure all
message frames are of appropriate length. The SENT_UPDATE parameter sets format.
Nibbles: 1
Quantity of ticks: 12 minimum (length determined by SENT_UPDATE option and by the
Syntax
individual structure of each SENT message)
Quantity of bits: n.a.
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Data Nibble Format
When transmitting normal operation data, information about the
magnetic field is embedded in the first three Data nibbles (see
figure 5). Each Data nibble consists of 4 bits with values ranging
from 0 to 15. In order to present an output with the resolution of
12 bits, 3 Data nibbles are required. The Data nibble containing
the MSB of the whole Data section is sent first.
Three additional optional Data nibbles can be associated with
other parameters, by setting the parameter SENT_DATA:
• Counter – Each message frame has a serial number in each
Counter nibble.
SENT_DATA = 0 0
Data Nibble 1,2,3: Magnetic field
Data Nibble 4,5: Counter
Data Nibble
6: Inverted 1 (default)
• Temperature – Temperature data from the device internal temperature sensor, in two’s complement format, with MSB first:
▫All zeros = 25°C.
▫Temperature slope is always 0.8 LSB/°C, except for serial
output protocol.
▫For serial output protocol, temperature slope = 0.5 LSB/°C.
• Inverted – The last nibble in the message frame is the first
nibble, inverted (as an additional error check).
1
2
3
Device output (12 bits)
SENT_DATA = 0 1
Data Nibble 1,2,3: Magnetic field
Data Nibble 4,5: Counter
Data Nibble
6: (zeros)
1
2
1
2
3
1
2
4
3
4
6
Inverted data nibble 1
5
Message counter (8 bits)
Device output (12 bits)
SENT_DATA = 1 0
Data Nibble 1,2,3: Magnetic field
Data Nibble 4,5,6: (skipped)
5
Message counter (8 bits)
Device output (12 bits)
SENT_DATA = 1 0
Data Nibble 1,2,3: Magnetic field
Data Nibble 4,5,6: Temperature
4
6
(zeros)
5
6
Temperature (12 bits)
3
Device output (12 bits)
Figure 5. Options for SENT messages from the device (Slave), determined by the SENT_DATA field programmed value
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Output Message Transfer
In the output stage of the sensor IC, signal samples proportional
to the magnetic information are latched into the SENT converter
and transferred to the user. The timing relationship, between the
moment when magnetic information is latched into the SENT
converter and the moment when the SENT message is transmitted
to the user, falls into two types of SENT message transfers:
• Synchronous message transfer. Each SENT message is transmitted after new magnetic information (or multiples of magnetic
information) reaches the SENT converter.
• Asynchronous message transfer. SENT messages are transmitted continuously, one after the other, not waiting for new magnetic information.
• Asynchronous message transfer with variable SENT message
duration—the device default state (SENT_UPDATE = 0). The
output stage transmits the SENT messages independently of
device internal output update rate (see figures 6 and 7). Allows
Status and
Communication
12-27 ticks
• Asynchronous triggered message transfer (SENT_UPDATE = 3
or 4).
Data
Nibble 1
12-27 ticks
Synchronous Transfer Modes
(SENT_UPDATE = 2)
Latching Point:
The last internal Output Update
sample available before this time
is latched into the Data nibbles for
the next SENT message transfer.
• Asynchronous data transfer with constant SENT message
duration (SENT_UPDATE = 1). The output stage transmits the
SENT messages independently of device internal output update
rate (see figures 6 and 8). The Pause pulse is always inserted
with a minimum nibble length of 12 ticks, but the nibble length
is increased if the message is shorter than the maximum message length.
• Synchronous data transfer (SENT_UPDATE = 2) where the
SENT message frame transmission rate is synchronized with
the device internal output update rate (set by BW value) (see
figures 6 and 9). If a particular message is shorter, a Pause pulse
is inserted with a length that completes the message period.
The SENT_UPDATE parameter determines the message transfer
state:
Calibration and
Synchronization
Pulse
56 ticks
message frame duration to vary according to the contents; no
Pause pulse is applied.
Data
Nibble 2
12-27 ticks
…
Data
Nibble n
12-27 ticks
CRC
12-27 ticks
Asynchronous Transfer Modes
(SENT_UPDATE = 0, SENT_UPDATE = 1)
Latching Point:
The last internal Output Update
sample available before this time
is latched into the Data nibbles for
the next SENT message transfer.
Figure 6. Latching Points for Available Data for SENT Message Data Nibbles
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TOUT
SENT message 2
TSENT2
SENT message 3
TSENT3
n+5
n+6
(data
skipped)
data
stat
sync
(data
skipped)
CRC
data
stat
n+4
sync
CRC
sync
CRC
SENT message 1
TSENT1
n+3
data
n+2
(data
skipped)
stat
n+1
data
sync
SENT
message
stat
n
Output stage
update with
magnetic
information
SENT message 4
TSENT4
Panel 7(a). TOUT < TSENTx (some data is not transmitted)
TOUT
n
SENT
message
n+1
SENT message 1
TSENT1
SENT message 2
TSENT2
SENT message 3
TSENT3
data
stat
sync
CRC
data
stat
sync
(old data
repeated)
CRC
data
stat
sync
CRC
data
stat
(old data
repeated)
sync
Output stage
update with
magnetic
information
SENT message 4
TSENT4
Panel 7(b). TOUT > TSENTx (some data is repeated)
Figure 7. Messages do not contain a Pause pulse (SENT_UPDATE = 0), so the SENT message frame rate is not constant. The
value transmitted in a message is taken from the last internal update ready before the first Data nibble of the message is composed.
Therefore, individual internal updates may be skipped (panel a) or repeated (panel b), depending on the BW bandwidth and the
message length defined by the SENT_TICK parameter setting.
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TOUT
SENT message 1
TSENT1 + TPAUSE1
SENT message 2
TSENT2 + TPAUSE2
n+5
n+6
sync
pause
(data
skipped)
CRC
CRC
data
stat
sync
CRC
data
stat
sync
pause
SENT
message
n+4
(data
skipped)
data
n+3
(data
skipped)
stat
n+2
sync
n+1
(data
skipped)
pause
n
Output stage
update with
magnetic
information
SENT message 3
TSENT3 + TPAUSE3
Panel 8(a). TOUT < TSENT + TPAUSE (some data is not transmitted)
TOUT
SENT message 1
TSENT1 + TPAUSE1
SENT message 2
TSENT2 + TPAUSE2
SENT message 3
TSENT3 + TPAUSE3
SENT message 4
TSENT4 + TPAUSE4
pause
CRC
data
stat
sync
pause
CRC
data
stat
(old data
repeated)
sync
pause
CRC
data
(old data
repeated)
stat
CRC
data
stat
sync
CRC
data
stat
pause
sync
SENT
message
n+1
sync
Output stage
update with
magnetic
information
pause
n
SENT message 5
TSENT5 + TPAUSE5
Panel 8(b). TOUT > TSENT+ TPAUSE (some data is repeated)
Figure 8. A constant message frame rate is used, and for each message, a Pause pulse is used to extend the message to match the
frame rate (SENT_UPDATE = 1). Internal updates may be skipped or repeated depending on the BW bandwidth and SENT message time
settings. The quantity of skipped (panel a) or repeated (panel b) internal updates can vary from message to message.
Note: Although the frame transmission rate is constant, discrete SENT messages do not represent equal time interval sampling of the
magnetic field.
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TOUT
TOUT
SENT
message
TSENT2 + TPAUSE2
Panel 9(c). (TOUT < TSENT +TPAUSE) The internal update
rate is the same as in panel (b), but the tick duration is
reduced slightly. The longest possible SENT message is
now synchronized at the internal update rate. Each update is
ready before the Synchronization nibble is composed, and is
transmitted. No updates are skipped.
SENT message 1
TSENT1 + TPAUSE1
data
stat
n+2
sync
data
(data
skipped)
CRC
pause
n+1
sync
data
stat
sync
CRC
pause
data
stat
SENT message 1
TSENT1 + TPAUSE1
sync
TOUT
Output stage n
update with
magnetic
information
n+1
sync
SENT
message
TSENT2 + TPAUSE2
Panel 9(b). (TOUT < TSENT +TPAUSE) The filter bandwidth
is reduced by twice relative to the bandwidth in panel (a),
which doubles the internal update interval. The longest
possible SENT message is now synchronized at two times
the internal update rate. The first update is ready before the
Synchronization nibble is composed, and is transmitted. Two
more updates occur before the next SENT message, so only
the second update data is included, and the one intervening
update is skipped.
TOUT
Output stage n
update with
magnetic
information
pause
SENT message 1
TSENT1 + TPAUSE1
TSENT2 + TPAUSE2
Panel 9(a). (TOUT < TSENT +TPAUSE) The longest possible
SENT message is synchronized at three times the
internal update rate. The first update is ready before the
Synchronization nibble is composed, and is transmitted. Three
more updates occur before the next SENT message, so only
the third update data is included, and the two intervening
updates are skipped.
CRC
data
stat
SENT
message
SENT message 1
TSENT1 + TPAUSE1
n+2
(data
skipped)
data
n+1
stat
n
sync
data
stat
(data
skipped)
CRC
Output stage
update with
magnetic
information
n+3
stat
SENT
message
data
sync
(data
skipped)
n+2
sync
n+1
pause
n
stat
Output stage
update with
magnetic
information
SENT
TSENT2 + TPAUSE2
Panel 9(d). (TOUT < TSENT +TPAUSE) The faster update rate
of panel (a) and the shorter tick duration of panel (c) are
applied. Because the panel (d) higher bandwidth setting also
applies, the overall device response time is faster than that
shown in panel (c). However, the panel (c) settings reduce
front-end noise better than those of panel (d), because of the
lower bandwidth.
Figure 9. The SENT message rate is synchronized with the internal device internal update rate. For each message, a Pause pulse
is used to extend the message to match the internal update rate (SENT_UPDATE = 2). A consistent number of updates are skipped
(panels a, b, and d) from message to message. The internal update value transmitted is from the last update ready before the
Synchronization and Calibration nibble of the message is composed.
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requires a prompt response on the current magnetic field.
The SENT_UPDATE parameter has two other options which
allow direct control of when magnetic field data is sent to the
external controller:
• Tandem data latching and sending (SENT_UPDATE = 3)
• Immediate data latching with a controllable delay before sending (SENT_UPDATE = 4)
When SENT_UPDATE = 3 (upper panel in figure 10), while the
sensor IC has a Pause pulse on the device output, the controller triggers a latch-and-send sequence by pulling the sensor IC
output low. When the controller releases the output, the last processed signal (proportional to the magnetic field) is latched into
the SENT converter, and after a delay of tdSENT, the latched data
is sent to the controller. This option is useful when the controller
When SENT_UPDATE = 4 (lower panel in figure 10), while the
sensor IC has a Pause pulse on the device output, the controller triggers a latch-and-send sequence by pulling the output low,
which immediately latches the last processed signal (proportional
to the magnetic field) into the SENT converter. This option
allows the controller to postpone receiving the data. When the
output is eventually released, the data is sent to the controller
after a delay of tdSENT . This option is useful where multiple sensor ICs are connected to the controller. All the sensor ICs can be
instructed at the same time to latch magnetic field data, and the
controller can then retrieve the data from each sensor IC individually.
Controller pulls OUT low
Controller releases OUT; magnetic data latched
SENT
messages
sync
pause
CRC
data
stat
Waiting
period, twait
sync
...
VOUTx
pause
tdSENT (6 ticks)
A1341 starts message containing latched data
...
...
(previous message)
SENT message
SENT_UPDATE = 3
Panel 10(a). SENT_UPDATE = 3
Controller pulls OUT low; magnetic data latched
Controller releases OUT
tdSENT (6 ticks)
SENT
messages
sync
pause
CRC
data
stat
Waiting
period, twait
sync
...
VOUTx
pause
A1341 starts message containing latched data
...
...
(previous message)
SENT message
SENT_UPDATE = 4
Panel 10(b). SENT_UPDATE = 4
Figure 10. Device output behavior where normal operation magnetic field data is latched at a defined time:
(panel a) if SENT_UPDATE = 3, latched and sent at end of a low pulse, or (panel b) if SENT_UPDATE = 4, latched
at the beginning of a low pulse, but not sent until the end of the pulse. The total delay from the beginning of the low
pulse until the data message begins is: twait + tdSENT .
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Optional Serial Output Protocol
In the Status and Communication section, the data format selection can be:
• Normal device output (voltage proportional to applied magnetic
field) in SENT protocol (SENT_SERIAL = 0).
• Augmented data on the magnetic parameters and device settings, in an optional Serial Output protocol (SENT_SERIAL =
1, 2, or 3). Any of these three protocols enables transmission of
values from the following EEPROM parameters, in the following order:
Message ID
(4 or 8 bits)
Data
(8, 12, or 16 bits)
0
Corrected temperature
1
SENS_COARSE
2
SIG_OFFSET
3
QOUT_FINE
4
SENS_MULT
5
CLAMP_HIGH
6
CLAMP_LOW
7
DEVICE_ID (134110 or 134310 , per device)
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▫ Additional Short serial protocol (SENT_SERIAL = 1). Has a
message payload of 12 bits: 8 bits are for value data, and 4 bits
for the message ID (identification). A total of 16 separate SENT
messages are required to transmit the entire data group.
▫ Additional Enhanced 16-bit serial protocol (SENT_SERIAL
= 2). Has 12 bits for value data, and 4 bits for the message ID. A
total of 18 SENT messages are required to transmit the entire data
group.
▫ Additional Enhanced 24-bit serial protocol (SENT_SERIAL
= 3). Has 16 bits for value data, and 8 bits for the message ID. A
total of 18 SENT messages are required to transmit the entire data
group.
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Device Response Time
The Device Response Time depends on three factors:
Table 3. Bandwidth Settings and Outcomes
• Propagation Delay – This is the traveling time of the signal
from the Input Stage of the Hall device to the Output Stage.
• Output Message Transfer – Synchronous and asynchronous.
• SENT Message Length – The various choices for the SENT
message configuration give different SENT message lengths.
These three factors are applied sequentially, as illustrated in
figure 11.
Propagation Delay and Output Update Rate
Propagation Delay and the Output Update Rate depend greatly
on the device internal filter bandwidth. The bandwidth is set by
programming the BW field in EEPROM. The correspondence of
programmed value with Propagation Delay and Output Update
Rate is given in table 3.
Programming
Code, BW
3-dB
Bandwidth
(kHz)
Maximum
Propagation
Delay
(ms)
Output Stage
Update
Frequency
(kHz)
0
1.5
0.63
8
1
3
0.37
16
2
1.5
0.63
8
3
0.750
1.26
4
4
0.375
2.52
2
5
0.188
5.04
1
6
0.094
10.08
0.500
7
0.047
20.16
0.250
Device Response Time
Propagation Delay
Magnetic Signal
Analog and Digital Signal Processing
of Input Magnetic Signal
(Internal Filtering)
TOUT
Output Update
K × SENT
Message Length
Output Stage
Conversion
to SENT
SENT Message
Figure 11. Model of Overall SENT Response Time. The summation of the significant processes is expressed
in the following equation:
Device Response Time = Propagation Delay + TOUT + K × SENT Message Length
where
TOUT is the period of the Output Update rate, and
K is the coefficient determined by the moment of the Output Stage update
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Asynchronous Transfer Minimum Response Time
The shortest device response time is realized when the Output
Update sample appears immediately before a new SENT message
is configured. In asynchronous mode, this can occur later in the
SENT message period, up to the Status and Communication bit,
as shown in figure 12.
Magnetic Field (B)
Input Signal
after Processing
Filter Delay
This is the sample with new
information that will be
transferred in the SENT message
Internal Output Update
Dat a
SENT Message
(Minimum Response)
Device Response Time
Figure 12. Minimum Device Response Time, Asynchronous Transfer mode
Minimum Device Response Time = Filter Delay + 3 × Data Nibble + CRC
Magnetic Field (B)
Input Signal
after Processing
Filter Delay
This is the sample with new
information that will be
transferred in the SENT message
Internal Output Update
Data
SENT Message
(Minimum Response)
Device Response Time
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Asynchronous Transfer Maximum Response Time
With Asynchronous Transfer selected, the longest device
response time is realized when an internal Output Update sample
appears immediately before the Filter Delay period ends, and
the next Status and Communication nibble ends before the next
sample occurs, as shown in figure 13. Latching of the sample
occurs near the end of the Status and Communication nibble.
This sample contains new
information but it comes
during Data nibbles
Magnetic Field (B)
Input Signal
after Processing
Filter
Delay
This sample contains new
information that is transferred
in the SENT message
TOUT
Internal Output Update
Data
Data
SENT Message
(Maximum Response)
TSENT
Device Response Time
Status and Communication nibble
Panel 13(a). TOUT < TSENT
This is the first sample containing
new information that is transferred
in a SENT message
Magnetic Field (B)
Input Signal
after Processing
Filter
Delay
TOUT
Internal Output Update
Data
SENT Message
(Maximum Response)
Data
TSENT
Status and Communication nibble
Device Response Time
Panel 13(b). TOUT > TSENT
Figure 13. Maximum Device Response Times Compared for TOUT < TSENT (panel a) and TOUT > TSENT (panel b)
Maximum Device Response Time = Filter Delay + TOUT + 3 × Data Nibble + CRC + TSENT
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Asynchronous Transfer with SENT Messages of
Equal Duration Maximum Response Time
With Asynchronous Transfer selected, and use of a Pause Pulse
is enabled to ensure all SENT messages are of the same duration,
the longest device response time is realized when an internal Output Update sample appears immediately before the Filter Delay
period ends, and the next Status and Communication nibble ends
before the next sample occurs, as shown in figure 14. Latching of
the sample occurs near the end of the Status and Communication
nibble.
This sample contains new
information but it comes
during Data nibbles
Magnetic Field (B)
Filter
Delay
Input Signal
after Processing
Internal Output Update
SENT Message
(Maximum Response)
This sample contains new
information that is transferred
in the SENT message
TOUT
Data
Data
Data
Data
P
P
P
TSENT
Status and Communication nibble
P indicates Pause Pulse
Device Response Time
Panel 14(a). TOUT < TSENT + TPAUSE
This is the first sample containing
new information that is transferred
in a SENT message
Magnetic Field (B)
Input Signal
after Processing
Filter
Delay
TOUT
Internal Output Update
SENT Message
(Maximum Response)
Data
P
P
Data
P
P
P
P
TSENT
Device Response Time
Status and Communication nibble
P indicates Pause Pulse
Panel 14(b). TOUT > TSENT + TPAUSE
Figure 14. Maximum Device Response Times Compared for TOUT < TSENT + TPAUSE (panel a) and TOUT > TSENT + TPAUSE (panel b).
Note: For purposes of comparison, the total length of the equal length SENT messages is for messages having the maximum
number of ticks in each section and a minimum Pause Pulse of 12 ticks.
Maximum Device Response Time = Filter Delay + TOUT + 3 × Data Nibble + CRC + TPAUSE + TSENT
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Magnetic Field (B)
Input Signal
after Processing
Filter Delay
This is the sample with new
information that will be
transferred in the SENT message
Synchronous Transfer Mode Minimum Response Time
Internal Output Update
The shortest device response time is realized when the Output
Update sample appears immediately before a new SENT message
SENT
is configured. In synchronous
mode,Message
this must occur simulta(Minimum Response)
neously with the start of the Synchronization bit, as shown in
figure 15.
Dat a
Device Response Time
Magnetic Field (B)
Input Signal
after Processing
Filter Delay
This is the sample with new
information that will be
transferred in the SENT message
Internal Output Update
Data
SENT Message
(Minimum Response)
Device Response Time
Figure 15. Minimum Device Response Time, Synchronous Transfer mode
Minimum Device Response Time = Filter Delay + TSENT
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Synchronous Transfer Maximum Response Time
With Synchronous Transfer selected, the longest device response
time is realized when an internal Output Update sample appears
immediately before the Filter Delay period ends, and the next
Status and Communication nibble ends before the next sample
occurs, as shown in figure 16. Latching of the sample occurs near
the end of the Status and Communication nibble.
This is the first sample with new
information that is transferred
in a SENT message
This is the first sample with new
new information
Magnetic Field (B)
Filter
Delay
Input Signal
after Processing
Internal Output Update
TOUT
Data
SENT Message
(Maximum Response)
Data
Data
PP
PP
PP
TSENT
Status and Communication nibble
P indicates Pause Pulse
Device Response Time
Panel 16(a). TOUT < TSENT + TPAUSE; Maximum Device Response Time = Filter Delay + TSENT + TPAUSE + TOUT
This is the first sample containing
new information that is transferred
in a SENT message
Magnetic Field (B)
Input Signal
after Processing
Filter
Delay
TOUT
Internal Output Update
SENT Message
(Maximum Response)
P
P
P
Data
Data
Data
PP
P
PP
P
TSENT
TPAUSE
Device Response Time
Status and Communication nibble
P indicates Pause Pulse
Panel 16(b). TOUT = TSENT + TPAUSE; Maximum Device Response Time = Filter Delay + TSENT + TOUT
Figure 16. Maximum Device Response Times Compared for TOUT < TSENT + TPAUSE (panel a) and
TOUT > TSENT + TPAUSE (panel b). Note: For purposes of comparison, the total duration of the equal SENT
messages is for messages having the maximum number of ticks in each section and a Pause Pulse of
12 ticks or more, satisfying the equation: TSENT + TPAUSE. = n × TOUT, where n is an integer number.
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Device Response Time with Continuous Field Change
In the case where the applied magnetic field is continuously
changing and the application requires the device output to track
the magnetic field closely:
• The Initial Response Delay can be treated the same as a device
response to a magnetic step function. The Initial Response Delay can be as long as the Maximum Response Time.
• After the Initial Response Delay, updates reflecting the continuous change are transferred with every SENT message.
These considerations are represented in figure 17. The Response
Delay for field A represents the minimum step response.
Filter Delay
Field B
Magn
0
eti
(B)
c Field
Field A
al H a
Intern
ll Sign
Initial Response Delay
to magnetic change
from zero field
al
Data nibbles
corresponding
to field A
Data nibbles
corresponding
to field B
One SENT message
tracking delay
Figure 17. Device Response Time Characteristics for Device in Continuously Changing Magnetic Field
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Fast SENT Feature
The Allegro proprietary programmable Fast SENT feature
includes:
• Minimum clock rate: 0.25 µs. This can be achieved by programming parameter SENT_TICK, register 7, bits 17:11, to code 1.
• Minimum quantity of fixed ticks for low-voltage interval:
4 ticks. This can be achieved by programming SENT_FIXED,
register 7, bits 10:9, to code 1.
• Number of Data nibbles: 3. This can be achieved by programming SENT_DATA, register 7, bits 4:3.
• Default update rate: One message after another, with no pause
pulse. Accept the default for SENT_UPDATE, register 7,
bits 7:5, default 0.
• Serial data: No serial data. Accept the default for
SENT_SERIAL, register 7, bits 1:0, default 0.
The Tick Time is set by programming the SENT_TICK field in
EEPROM. Tick Time is the internal 4 MHz count divided by the
SENT_TICK setting. The correspondence of programmed value
with Tick Time is given in table 4.
The shortest Tick Time is 0.25 µs. Given the Minimum Message
Length, as defined above, and the maximum ticks as shown in
figure 9, the shortest SENT message duration is:
Minimum Message Length = (56 ticks
+ 27 ticks
+ 27 ticks × 3
+ 27 ticks)
× 0.25
= 191 × 0.25 = 47.75 µs
Table 4. Tick Settings and Outcomes
0 (default)
3
Calibration and
Synchronization
Pulse
56 ticks
Status and
Communication
12-27 ticks
Data
Nibble 1
12-27 ticks
Data
Nibble 2
12-27 ticks
0.25
2
0.5
12
3
…
Minimum Message Length = (Synchronization and
Calibration Pulse
+ Status and Communication
+ Data Nibble × 3
+ CRC)
× Tick Time
1
…
Tick Time
(4 MHz / SENT_TICK)
(µs)
…
The shortest SENT message contains 6 sections, as illustrated in
figure 18. The shortest duration of a SENT message can be calculated using the following equation:
Programming Code
SENT_TICK
…
Minimum Message Length
127
31.75
Data
Nibble 3
12-27 ticks
CRC
12-27 ticks
Figure 18. Model of Shortest Valid SENT Message
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A comparison of the default SENT message transmission rate and
the Fast SENT rate is shown in figure 19. Approximately 12 Fast
SENT messages can be sent in the same time period as one message at the default rate.
An expanded view of one Fast SENT message is provided in figure 20. Including a payload of three data nibbles, the total elapsed
time is approximately 33 µs.
Single message at default SENT rate
Default SENT
Single tick duration = 3 µs
Fast SENT
Single tick duration = 0.25 µs
1
Fast SENT Messages
2
3
4
5
6
7
8
9 10 11 12
le
3)
ibb
C
CR
Da
ta
nib
ble
s(
nn
St
Co atus
mm an
un d
ica
tio
Sy
nc
hro
niz
ati
on
nib
ble
Figure 19. Comparison of time required to output (top) a default SENT mode message, and (bottom) a Fast SENT mode message
Single message at Fast SENT rate
Total duration ≈ 33 µs
Figure 20. Expanded view of a single Fast SENT mode message
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Device Response Time Example Calculation
A comparison of typical minimum and maximum device response
times is presented in table 5. The results are based on the following assumptions:
• Tick Time = 0.25 µs
• Bandwidth = 3 kHz
• Length of 4 data nibbles = 27 ticks (every nibble has the maximum number of ticks)
• Maximum Message Length TSENT = 47.75 µs (every SENT
message has the maximum message length, and each message
section has the maximum number of ticks)
• Maximum Device Response Time formula applied is for
condition where TOUT > TSENT ( + TPAUSE )
• TOUT = 1 / 16 kHz = 62.5 µs
• Internal Filter Delay = 350 µs
Table 5. Comparative Response Times
Asynchronous Transfer
(µs)
Asynchronous Transfer
with equal SENT Duration
(µs)
Synchronous Transfer
(µs)
Minimum Response Times
Internal Filter Delay + 4 data
nibbles = 350 + 27 = 377
Internal Filter Delay + 4 data
nibbles = 350 + 27 = 377
Internal Filter Delay + SENT
message = 350 + 47.75 = 397.75
Maximum Response Times
Internal Filter Delay +
SENT message + Synch pulse +
Status/Communication =
350 + 47.75 + 20.75 = 418.5
Internal Filter Delay +
SENT message + Synch pulse +
Status/Communication =
350 + 47.75 + 20.75 = 418.5
Internal Filter Delay + TOUT
= 350 + 62.5 = 412.5
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Trigger Mode Fast SENT Feature
Trigger mode can be applied to the Allegro Fast SENT feature by
setting the SENT_UPDATE field to 3 or 4. When the message
should be transmitted, the device output must be pulled low for a
minimum interval of 2 ticks, and then pulled high. After 6 ticks
have expired at the high level, the SENT message is transmitted.
In preparation for transmission, the sample data is latched at the
end of the Status and Communication nibble. It is then sent at the
beginning of a Trigger pulse (SENT_UPDATE set to 3) or at the
end of the pulse (SENT_UPDATE set to 4).
The actual response time depends on the relative timing of the
internal output update and the latching (figure 21):
• The minimum response time occurs when the output data was
latched immediately after a new internal sample emerged.
• The maximum response time occurs when the output data was
latched immediately before a new internal sample emerged.
Magnetic Field (B)
Internal Delay
Internal Output Update
TOUT
Dat
a
Data latched into
Data nibbles
SENT Message
(Minimum Response)
2 ticks at low +
6 ticks at high
SENT Message
(Maximum Response)
TSENT
Da
ta
Data latched into
Data nibbles
TOUT
2 ticks at low +
6 ticks at high
TSENT
Maximum Device Response Time
Figure 21. Sensor IC response characteristics using triggered Fast SENT mode
Calculation assuming following parameters:
• Tick time of 0.25 µs
• Message format of 3 data nibbles with maximum length of 27 ticks
• Device internal bandwidth of 3000 Hz
• Propagation delay of 350 µs
Minimum Response Time ≈ Propagation Delay + Length of 4 Data Nibbles = 350 + 27 = 377 µs
Maximum Response Time ≈ Propagation Delay + TOUT + Length of 4 Data Nibbles (3 Data and 1 CRC) = 350 + 62.5 + 27 = 439.5 µs
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Trigger Mode with Two Sensor ICs
Trigger mode can be applied to compare the simultaneous output
of two Allegro devices. This feature allows dual sources to be
used without any requirement to synchronize the clocking of the
Allegro devices.
The actual response time of each of the devices depends on the
independent relative timing of the internal sampling cycle and
the latching (see the Trigger Mode Fast SENT feature section).
If the two devices receive the Trigger pulse at the same time,
the internal timing can lead to a maximum difference defined by
the period of the output update signal between the actual sample
acquisitions of the two devices. As shown in figure 22, the effect
is that different sample points can be used for the data output.
Magnetic Field (B)
Signal after Processing
(Sensor IC 1)
TOUT
Data
SENT data from Sample 8
SENT Message
(Sensor IC 1)
Both Sensor IC
outputs released
at the same time
Signal after Processing
(Sensor IC 2)
TSENT
Data latched into
Data nibbles
TOUT
Data
SENT data from Sample 7
SENT Message
(Sensor IC 2)
TSENT
Figure 22. Sensor IC differential response characteristics using two Sensor ICs in Trigger mode
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Adjustable Nibble Fall Time
The timing of the nibble fall time can be adjusted by a combination of an external capacitor and the value programmed for the
OUTDRV_CFG parameter. The value of an external capacitor, CLOAD , on the the OUT pin sets the coarse range for the
fall time. Within that range, a fine setting is determined by the
OUTDRV_DFG programmed code, according to table 6.
Table 6. Nibble Fall Time Values (OUTDRV_CFG)
Fall Time (Typical)
(µs)
Code
Values
000 (Default)
001
010
011
100
101
110
111
CLOAD = 100 pF
CLOAD = 1 nF
CLOAD = 10 nF
0.048
0.114.
0.202
0.290
0.760
1.539
3.161
4.819
0.149
0.217
0.309
0.400
0.854
1.555
2.978
4.442
1.324
1.323
1.404
1.492
1.948
2.669
4.118
5.557
NOTE: Values are based on design simulations. Lower values have been obtained in actual benchtop tests.
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Electrical Specifications
Typical Allegro device specifications are given in table 7.
Table 7. OPERATING CHARACTERISTICS Valid through full operating temperature range, TA , and supply voltage, VCC ,
CBYPASS = 10 nF, unless otherwise specified
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
General Electrical Characteristics1
SENT Message Duration
Minimum Programmable SENT
Message Duration
tSENT
Tick time = 3 µs, 3 data nibbles of
information, nibble length = 27 ticks
–
573
–
µs
tSENTMIN
Tick time = 0.25 µs, 3 data nibbles of
information, nibble length = 27 ticks
–
47
–
µs
VSENT(L)
10 kΩ ≤ Rpullup ≤ 50 kΩ
SENT Programmable Characteristics1
SENT Output Signal2,3
SENT Output Trigger Signal
VSENT(H)
–
–
0.05
V
Minimum Rpullup = 10 kΩ
0.9 × VCC
–
–
V
Maximum Rpullup = 50 kΩ
0.7 × VCC
–
–
V
VSENTtrig(L)
–
–
1.2
V
VSENTtrig(H)
2.8
–
–
V
1 Determined
by design.
2 For pull-up values lower than 10 kΩ, V
SENT(L) will be higher and can be calculated as: VSENT(L) = VPULL-UP × [60 (Ω) / (60 (Ω) + RPULL-UP) ]. Therefore,
for RPULL-UP = 500 Ω, and VPULL-UP = 5 V, low voltage will be a minimum of 535 mV.
3 For pull-up values lower than 10 kΩ, V
SENT(H) will be higher than 0.9 × VCC .
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SENT Data Programming Parameters
OUTDRV_CFG (Register Address: 0x07, bits 20:18)
Function
Syntax
Related Commands
Output Signal Configuration
Sets configuration of the output signal slew-rate control. Sets the ramp rate on the gate of
the output driver, thereby changing slew rate at the output.
Quantity of bits: 3
–
Fall Time (Typical)
(µs)
Code
000 (Default)
001
010
011
100
101
110
111
Values
Options
Examples
CLOAD = 100 pF
CLOAD = 1 nF
CLOAD = 10 nF
0.048
0.114.
0.202
0.290
0.760
1.539
3.161
4.819
0.149
0.217
0.309
0.400
0.854
1.555
2.978
4.442
1.324
1.323
1.404
1.492
1.948
2.669
4.118
5.557
NOTE: Fall Time values are based on design simulations. Lower values have been obtained
in actual benchtop tests.
–
SENT_DATA (Register Address: 0x07, bits 4:3)
Function
Syntax
Related Commands
Values
296108-AN
Data Nibble Format
Quantity and contents of Data nibbles in message. (Does not relate to data contained in the
Status and Communication nibble.)
Quantity of Bits: 2
–
0 0: Nibbles 1,2,3: magnetic field data; nibbles 4,5: counter data;
nibble 6: inverted nibble 1 (Default)
0 1: Nibbles 1,2,3: magnetic field data; nibbles 4,5: counter data;
nibble 6: all zeros
1 0: Nibbles 1,2,3: magnetic field data; nibbles 4,5,6: current temperature data
1 1: Nibbles 1,2,3: magnetic field data (nibbles 4,5,6 skipped)
Options
–
Examples
–
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SENT_FIXED (Register Address: 0x07, bits 10:9)
Function
Syntax
Related Commands
Values
Options
Examples
Fixed Interval Duration
Indicates the quantity of ticks in fixed-duration intervals.
Quantity of Bits: 2
SENT_LOVAR
0 0:
0 1:
1 0:
1 1:
5 ticks (Default)
4 ticks
7 ticks
8 ticks
SENT_FIXED = 1 (4 ticks) does not meet the SENT spec, but is provided for custom fast or
improved-EMI communication.
–
SENT_LOVAR (Register Address: 0x07, bit 8)
Function
Syntax
Related Commands
Quantity of Bits: 1
SENT_FIXED
Values
0: Low interval of every nibble is fixed in duration, and the high interval becomes the
information state (Default).
1: High interval of every nibble is fixed in duration, and the low interval becomes the
information state.
Options
SENT_LOVAR = 0 meets the SENT specification.
SENT_LOVAR = 1 does not meet the SENT spec, but is provided for custom improved-EMI
communication.
For SENT_UPDATE = 3 or 4, the Pause pulse has a fixed low time regardless of
the SENT_LOVAR setting.
Examples
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State Assignments
Assigns fixed duration state (becomes delimiting state; other interval becomes the
information state)
–
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SENT_SERIAL (Register Address: 0x07, bits 1:0)
Function
Syntax
Related Commands
Status and Communication Nibble Format
Defines values of bits 2 and 3 inside the Status and Communication nibble.
Quantity of Bits: 2
–
0 0: Bits 2 and 3 are 0 (Default).
0 1: Bits 2 and 3 are 0 part of the Short Serial protocol: 8-bit value data, 4-bit message ID,
16 SENT frames are required to send an entire serial message.
1 0: Bits 2 and 3 are part of the Enhanced 16-bit Serial protocol: 12-bit value data, 4-bit
message ID, 18 SENT frames are required to send an entire serial message.
1 1: Bits 2 and 3 are part of the Enhanced 24-bit Serial protocol: 16-bit value data, 8-bit
message ID, 18 SENT frames are required to send an entire serial message.
Values
Options
–
Examples
–
SENT_STATUS (Register Address: 0x07, bit 2)
Function
Syntax
Related Commands
Values
Error Condition Status
Defines values of bits 0 and 1 inside the Status and Communication nibble.
Defines data inside the Status and Communication nibble on device error status.
Quantity of Bits: 1
SENT_SERIAL
(SENT_STATUS = 0)
0 0: No error (Default)
0 1: Not used
1 0: Overvoltage condition
1 1: Nonrecoverable EEPROM error, bad Linearization table or other error
(SENT_STATUS = 1)
0 0: No error (Default)
0 1: Error condition
Options
Examples
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–
A Status and Communication nibble value of 0010 indicates an overvoltage condition.
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SENT_TICK (Register Address: 0x07, bits 17:11)
Function
Syntax
Related Commands
Tick Duration
Sets the SENT tick rate coefficient: 4 MHz / SENT_TICK = tick (µs)
Quantity of Bits: 7
Any value from 0 to 127 can be used
–
PWM Frequency (Typical)
(µs)
Code
000 0000
000 0001
000 0010
000 0111
111 1111
111 1110
111 1111
Values
Options
Examples
3.0 (Default)
0.25
0.5
0.75
32
31.5
31.75
Coefficient
(MHz/SENT_TICK)
4/12
4/1
4/2
4/3
4/125
4/126
4/127
SENT_TICK = 1 through 11 do not meet the SENT spec, but are provided for custom fast
communication.
–
SENT_UPDATE (Register Address: 0x07, bits 7:5)
Function
Syntax
Related Commands
Values
296108-AN
Pause Pulse and Frame Rate
Pause pulse usage and message frame rate.
Quantity of Bits: 3
SENT_LOVAR
000: No Pause pulse; new frame immediately follows previous frame (Default).
001: Pause pulse used for minimum constant frame rate (Length of other message
sections, plus length of Pause Pulse nibble, is constant. For the maximum message
length, Pause pulse information state is the minimum size of 12 ticks.)
010: Pause pulse used for constant frame rate, synchronized with device internal update
rate. (Handshaking occurs such that the Synchronization and Calibration nibble starts
immediately after the next new data word is ready.)
011: Pause pulse held indefinitely until receipt of trigger pulse (OUT pulled low) from the
controller, data latched after output released and message is sent.
100: Pause pulse held indefinitely until receipt of trigger pulse (OUT pulled low) from the
controller, data latched immediately and sent when output is released.
101, 110, 111: Same function as 000.
Options
–
Examples
–
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Copyright ©2015, Allegro MicroSystems, LLC
The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the
customer with respect to the subject matter of this document. The information being provided does not guarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes. It is the customer’s responsibility to do sufficient qualification
testing of the final product to insure that it is reliable and meets all design requirements.
For the latest version of this document, visit our website:
www.allegromicro.com
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