SENT messages are encoded and decoded
based on the time between falling or rising edges, depending on the idle polarity.
For this section, the high idle polarity is described as an example. The same
information applies for low idle polarity, given the edge direction of the clock is
swapped. The messages are timed using clock tick time, or TT, which is calculated
during the calibration/synchronization pulse.
A slow frame is composed of multiple fast frames, and transmits longer streams of
data. The slow frame also utilizes multiple fast frames to transmit additional
information through the status and communication nibble, such as the message ID,
slow data, and CRC.
The frame format for SENT is as
follows:
- Calibration or synchronization
pulse with a fixed period (56 clock ticks)
- This is used by the
receiver as a reference to measure the TT. The pulse starts with a
falling edge for high idle polarity and rising edge for low idle
polarity and the counter for the synchronization period starts counting.
For a high idle polarity, the period between the first falling edge and
the second falling edge is divided into 56 to determine the TT. The TT
is used to sample the nibbles' values. If more than one synchronization
pulse is detected, the first one is ignored under the assumption the
pulse is a pause pulse.
- One status and serial
communication nibble pulse (12 to 27 clock ticks)
- The status nibble
transmits miscellaneous information such as part numbers, mode of
operation, and error code information in slow channel mode (1-bit per
frame serial message). This nibble is not included for the frame's CRC
calculation if the RX_CRC_WITH_STATUS bit is not set.
Table 42-3 Status Nibble
Description
Bit |
Functionality |
0 |
Channel 2 error indicator (1 = error) |
1 |
Channel 1 error indicator (1 = error) |
2 |
Serial data message bit (CRC, message ID, and
data) |
3 |
Short Serial Message Pattern: Start with 1 and
the rest of the 15 messages are 0
Enhanced Serial Message Pattern: Unique pattern
[0]1111110, message 13 and 18 are 0; the [0] is a
zero pre-condition before the start of serial
message 1
|
- The status nibble
for the FAST channel without a slow serial message is 0, with an
option for using bit 0 and/or bit 1 as an error flag for certain
sensors. If the sensor is in error, bit 0 or 1 of the status
nibble is set to 1.
- A Short or
Enhanced Serial Message is ignored on the status and
communication nibble until the appropriate pattern of the
message is received. Note that if the status nibble is used in a
way not defined by the J2716 standard, then mask the serial
errors appropriately.
- Sequence of one to eight data
nibble pulses (12 to 27 clock ticks for each nibble pulse)
- For high idle polarity,
the pulse starts with a falling edge and remains low for a few clock
ticks. Then each nibble's value is determined between falling edges,
with the minimum pulse period being 12 clock ticks and the maximum being
27 clock ticks. The incoming serial nibble value is sampled at a clock
tick length of TT / 2. The sampling occurs at the middle of the nibble
pulse. For high idle polarity, the SENT TT counter counts the number of
TT between two falling edges of the nibble pause and offsets the value
with 12.
- If the number of TT
between the two falling edges is less than 11 or greater than 27, the
value is invalid and the frame error is set.
- One CRC nibble pulse (12 to 27
clock ticks)
- See Section 42.3.2.
- Pause pulse (optional)
- This is an optional part
of the SENT message format. For a fixed-length frame, the pause pulse
fills up the message after the checksum nibble. This is the portion
between 12 clock ticks and 768 clock ticks. If the RX_PPENB is enabled
in the RCFG register the receiver expects the next pulse to be a pause
pulse after the CRC nibble. When no pause pulse is enabled, the receiver
expects that a packet received is immediately followed by another packet
calibration/synchronization pulse. This does not apply for MTP_MODE,
where an end pulse is expected at the end of the final CRC field.
- End pulse for synchronous mode
SENT that requires MTPG (12 ticks)
- For sensors synchronously
responding to the MTPG, a 12-clock tick end pulse terminates the frame
transmission after the CRC nibble and idly waits for the next trigger
pulse. The SENT peripheral does not require a complete end pulse, and
acknowledges the received packet when the CRC is verified.