What Is SENT SPC?

SENT SPC interrogation mechanism showing 
MTP preceding standard SENT frame.

Single Edge Nibble Transmission protocol, more commonly known as SENT (SAE J2716 JAN201604), has long been used by the Automotive industry to report low-speed sensor data to the Engine Control Unit (ECU). SENT was developed because the environment in a car was too noisy to transmit high resolution (10- or 12-bit) sensor data vertically on a 5 V bus. Instead, sensor signals are transmitted as a series of pulses, with data measured by falling-edge to falling-edge times. Information lies within the width of the pulses. Later specifications of SENT introduced Fast and Slow Channels to designate different streams of information carried within the same messages.

To address the exponential increase in the number of sensors within a single vehicle, Infineon specified SENT Short PWM Code (SPC), a half-duplex variation of SENT that allows a single Master microcontroller to manage up-to-four Slave sensors on the same wire. SENT SPC achieves this by introducing a new element into the SENT frame definition: the Master Trigger Pulse (MTP). The latest release of our SENT TDME software can decode SENT SPC traffic including the MTP, as well as legacy SENT traffic.

Because the pace and order of transmission is determined by the controller, SENT SPC enables engineers to design systems where information can be requested at specific times or with different frequencies, unlike SENT’s continuous broadcast. The controller can interrogate sensors with higher priority more often, and sensors with lower priority less often. By reducing redundancies in ECU design, SENT SPC also coincidentally helps to lower costs.

While the MTP makes it rather easy to identify from which sensor a message originates, as with any pulse-width modulated system, there is still the problem of verifying that the MTPs remain within tolerance.

How It Works

The SENT physical layer consists of a signal line, a +5 V supply voltage line, and a ground. A level of < 0.5 V on the signal line represents a logic-low, while > 4.1 V is a logic-high. 

Each sensor is assigned a unique MTP length, which acts like an address. The controller interrogates a sensor by emitting a logic-low MTP of the requisite length, then relinquishes the line for the sensor to reply. Only the sensor interrogated is expected to reply. 

The microcontroller can interrogate different sensors by varying the length of the MTP. While current implementations usually follow a “round robin” interrogation, with sensors 0 through 3 polled in order (0,1,2,3…), it is entirely possible for any sequence to be established (e.g., 0,1,0,2,0,3…). 

SENT SPC Frame Structure

SENT SPC frame decoded into nibbles, showing the MTP at the start,
the SYNC, five DATA nibbles and PP at the end.

The frame begins with the Master Trigger Pulse, which acts like an address. Each sensor (0 through 3) is assigned a unique MTP length, which is defined by the specification as a multiple of the Tick Time; for example, the MTP of sensor 0 must be between 7 and 15 ticks. Following the MTP is a latency prior to the sensor reply. In Teledyne LeCroy decoders, this time is shown as the Sensor Response Time.

Otherwise, there are no structural differences between a message broadcasted continuously by a SENT sensor and a message emitted upon request by a SENT SPC sensor. The following fields are present in all SENT frames, although their length may vary.

The Synchronization Pulse (SYNC) establishes the Tick Time “clock” used to recover the number of ticks, and therefore nibbles and words, represented by the length of the data pulses. TT is always computed at 1/56th of the SYNC length.

The Data payload contains the actual information. This may be anywhere from five to eight nibbles, each representing four bits of data.

Each nibble starts with a logic-low period of at least 5 ticks, followed by a logic-high period representing the encoded data value: 12 ticks for binary 0000, 13 ticks for binary 0001, up to 27 ticks for binary 1111. In the Teledyne LeCroy SENT decoders, SENT nibbles can be decoded individually or grouped into 8-bit words.

The first nibble is a Status and Communication Nibble that consists of four bits of additional information used by the CRC:   

• Bit 0 Start Bit
• Bit 1 Serial Data Bit
• Bit 2 Optional Slow Channel Data
• Bit 3 Optional Slow Channel Data

The final nibble of the payload is always a Cyclic Redundancy Check to confirm that the clocking is working correctly. Different CRC methods have been defined in each version of the specification.

In the 2010 and 2016 SENT specifications, an optional Pause Pulse (PP) separates message frames. In earlier specifications, this separator was known as the Interframe Gap.