24.2 Architectural Overview

SENT messages are encoded and decoded based on the time between falling edges. The protocol’s timing is based on a predetermined time unit, T_TICK, which can vary from 3 to 90 μs. Both the transmitter and receiver must be preconfigured for the same value of T_TICK. The SENT specification allows messages to be validated with up to a 20% variation in T_TICK. This allows for the transmitter and receiver to run from different clocks that may be inaccurate and drift with time and temperature.

An overview of the SENT module is shown in Figure 24-1.

The period from the start of the Sync period to the end of the CRC nibble comprises the message frame. When the optional Pause period is present, this makes one SENT message with a length of T_FRAME (generally expressed in μs). Figure 24-2 shows the construction of a typical six-nibble data frame, with the numbers representing the minimum or maximum number of tick times for each section.

The Sync period starts the message frame and is used for synchronization of T_TICK between the transmitter and receiver. When configured for Transmit mode, the module drives the line low for five ticks and to a High-Impedance state for 51 ticks.

A four-bit status nibble follows the Sync pulse and may be used for device status, identification or alternatively, used as additional data. The status nibble is formatted the same as a data nibble.

After the status nibble is one or more (up to six) data nibbles. These are four bits in length and are encoded as the data value plus 12 ticks. This yields a minimum value of 12 ticks for 0h and a maximum value of 27 ticks for Fh. When configured for Transmit mode, the module drives the line low for five ticks and into a High-Impedance state for the remaining 7 to 22 ticks.

The CRC data nibble follows the data payload. This is a 4-bit CRC of the six data nibbles only. The CRC is calculated using a polynomial of, x⁴ + x³ + x² + 1, with a seed value of ‘0101’. It is then padded with ‘0’ to help detect shift errors. The CRC nibble is formatted the same as a data nibble.

Since the data values are encoded in the time between falling edges, the SENT protocol may produce a variable length message. In some applications, the Pause pulse period is used to pad the message length so that messages will always be received at a constant time interval. The module provides support to automatically calculate the Pause duration needed for periodic transmissions. When configured for Transmit mode, the module drives the line low for five ticks and into a High-Impedance state for the remaining Pause time.