How to test the automotive SENT protocol

Oscilloscopes and dedicated software can help analyze SENT data.

In “What is the automotive SENT protocol?” we looked at the Single Edge Nibble Transmission (SENT) protocol, as defined in accordance with the Society of Automotive Engineers (SAE) J2716 standard. We reviewed the SENT message frame, which includes a synchronization/calibration pulse followed by a status nibble, six data nibbles, a CRC nibble, and an optional pause pulse.

How do we test SENT?
Figure 1 shows the basic SENT message frame as a function of time t. The frame begins on a falling edge (t = 0), often the falling edge of a previous message frame’s pause pulse. All the information in the message frame depends on the timing of subsequent falling edges relative to t = 0. One of the first measurements we can make is the timing of the synchronization/calibration (synch) pulse’s falling edge relative to t = 0. In Figure 1, that point is at 168 µs. In “What is the automotive SENT protocol?” we noted that the standard defines the pulse’s width from falling edge to falling edge as 56 tick times, so we can divide 168 µs by 56 to learn that one tick is 3 µs.

Figure 1. SENT data resides in the timing of successive falling edges.

How do we decode the data?
Recall that the nibble widths (from falling edge to falling edge) range from 12 to 27 ticks, corresponding to binary values from 0000 to 1111. Table 1 lists all 16 values.

Table 1. SENT nibble widths and corresponding values.

Now, we can use an oscilloscope to manually measure the timing of each nibble’s falling edge relative to t = 0. Table 2 lists the times in microseconds and ticks as well as the difference between successive falling edge times (labeled Dt). We can compare the Dt values with Table 1 to obtain the binary values, shown in the right-hand column of Table 2.

Table 2. SENT message-frame timing.

Can we automate this process?
Yes, many oscilloscope vendors offer SENT software tools with their instruments, sometimes as part of an automotive test option that also supports other automotive buses such as CAD, CAN FD, and LIN. These tools offer features such as hex and binary decoding of fast and slow SENT channels. Many also offer symbolic decoding, displaying physical parameters such as temperature and pressure. In addition, they offer flexible triggering options that let you trigger various errors, including CRC and timing errors, and support multiple CRC calculations. Oscilloscopes with hardware-based decoding can increase your chances of capturing intermittent errors.

You commented, “What is the automotive SENT protocol?” SENT is a one-way protocol supporting communication from the sensor to the controller, but you added that the half-duplex SENT Short PWM Code (SPC) version lets a controller communicate with multiple sensors. Do oscilloscope makers support that version?
Yes, tools are available that support SENT SPC, which allows a controller to communicate with up to four sensors (Figure 2). With SENT SPC, the controller can use the signal line to instruct a particular sensor to respond. The controller might, for example, query sensor 0 more frequently than the other three if the sensor 0 reading is more critical to the process being monitored.

Figure 2. SENT SPC enables a controller to communicate with up to four sensors.

For the controller to specify a particular sensor, SENT SPC adds a prefix to the SENT message frame of Figure 1. That prefix begins with what’s called the Master Trigger Pulse (MTP). The controller initiates the MTP by driving the data line low. The controller will release the line after a period ranging from seven to 82 ticks, where the number of ticks corresponds to which sensor should respond — for example, seven to 15 ticks for sensor 0. Some SENT oscilloscope tools let you specify custom tick ranges for each sensor. After the MTP, the data line goes high during an interval called the sensor response time, after which the selected sensor initiates a synch pulse, and data transmission occurs as it would in a normal SENT transmission. Figure 3, captured by engineers at Teledyne LeCroy using the company’s SENT decoder tools, shows a complete SENT SPC message frame with a 32.5 µs MTP and 48.89 µs sensor response time, followed by a synch pulse, five nibbles, and the beginning of a pause pulse.

Figure 3. This SENT SPC message frame begins with an MPT followed by a sensor response time, synch pulse, five nibbles, and a pause pulse (Image: Teledyne LeCroy).

Wait, the synch pulse appears to be only 27.35 µs. Doesn’t that make for a very short tick time?
Yes, semiconductor companies are always pushing the envelope of what original standards call for, and today, you can buy chips that support SENT tick times down to half a microsecond or less.

Where can I learn more about testing SENT?
Quickly Characterize Automotive Serial Buses (Keysight Technologies)
SENT SPC serial bus decoding (Pico Technology)
SENT oscilloscope software (Rohde & Schwarz)
Debugging SENT Automotive Buses with an Oscilloscope (Tektronix)
SENT Basics: Verifying a Stable MTP for SENT SPC (Teledyne LeCroy)