The Single-Edge Nibble Transmission (SENT) protocol transports highresolution sensor data for temperature, pressure, throttle position, and mass airflow in automotive applications. SENT is a unidirectional protocol supporting communications from smart sensors to an ECU. Meant to compete with the PSI5 protocol, SENT is a simpler and lowercost alternative to CAN and LIN networks. Teledyne LeCroy oscilloscopes can be equipped with software-based decoders for a large number of industry standard serial data protocols. The following describes using Teledyne LeCroy’s SENT configurable decode software for decoding SENT signals.
About the SENT Physical and Data Link Layers
Before examining the decoding of the specific signal used for this document, it may be instructive to cover some of the basic information regarding the SENT specification. One may examine the base specification in more detail at: www.sae.org.
The SENT protocol is a one-way, asynchronous voltage interface requiring three wires: a signal line (low state < 0.5V, high state > 4.1V), a supply voltage line (5V), and a ground line.
A SENT message (Figure 1) consists of:
- A calibration/synchronization pulse with a period of 56 clock ticks
- One 4-bit status and serial communication nibble pulse of 12 to 27 clock ticks
- A sequence of from one to six 4-bit data nibble pulses (12 to 27 clock ticks each) representing the values of the signal(s) to be communicated
- The number of nibbles is fixed for each application of the encoding scheme (i.e. throttle position sensors, mass air flow, etc.) but can vary between applications. For example, if two 12-bit values are transmitted, six nibbles will be communicated.
- One 4-bit checksum nibble pulse of 12 to 27 clock ticks
- One optional pause pulse
Embarking on a Decode
Start by acquiring a signal. For purposes of this document, the displayed signal is a Hall sensor output with a tick time of 1.5 µs and IdleLow (Figure 2).
Once the signal is acquired and suitably displayed on the oscilloscope screen, use the Analysis drop-down menu at the top of the screen to access the Serial Decode dialog box, as shown in Figure 3.
Pressing the Protocol button in the Decode 1 row brings up a popup menu of different protocols (what appears in the menu is determined by which serial-decode software options are installed in the oscilloscope). The decode options are shown in Figure 4:
Select SENT in the Protocol popup menu. Next, designate the source of the on-screen trace in this case, the trace is stored in Memory 1 or M1. The decoder works on all channels, math traces, and memory traces. In the Serial Decode dialog box, assign the on-screen trace to Decode 1 by turning Decode 1 on. Then press the Setup button for Decode 1, which opens the Decode Setup tab in the dialog box.
The Four Tabs: Basic, Fast Channel, Slow Channel, and Levels
Upon entering the Decode Setup tab, note its layout as shown in Figure 5:
Be sure to check the View Decode box. Note that Source 1 (Data) is set to M1 and the Protocol to SENT. The Basic tab will be open on the right by default. Try switching between the Basic, Fast Channel, Slow Channel, and Levels tabs. Then return to the Basic tab for the next section of the tutorial.
The Basic Tab
The Basic tab, shown at right in Figure 5, provides all of the fundamental controls required to allow proper bitlevel decoding. In the Viewing section of the tab, Decode Type selects whether to decode Nibbles or Words in the signal. The Channels button allows a choice of decoding Fast Only, Slow Only, or Both channels. The Format button chooses whether to decode in Hexidecimal or Decimal formats.
In the Physical Layer section of the tab, the TickTime button specifies the time in seconds between a nibble of value N and a nibble of value N+1. The TickTime Tolerance button defines how the CAL pulse is filtered with respect to the tick time entered by the user. The Idle State button defines where the idle state lies: IdleLow or IdleHigh. Use the Nibbles button to determine how many nibbles make up a word (eight is the default value).
In the Protocol Details section of the Basic tab, the Version button provides a choice of the protocol used in the decoded signal: February 2008 or January 2010. If selecting the latter, the user may choose to enable or disable other protocol details. Checking the New CRC button invokes CRC computation as per 2010 guidelines. Unchecking it selects CRC computation using the 2008 guidelines. Checking the Pause Pulse button tells the decode algorithm to expect a Pause pulse as per the 2010 recommended implementation. The Pause pulse follows the CRC of message N and precedes the CAL pulse of message N+1.
Shown in Figure 6 is a screen capture of the signal in Figure 2 with SENT decoding invoked. The upper grid is the signal trace itself, while the lower grid displays a zoomed view of the third burst from left.
The Fast Channel Tab
The Fast Channel tab appears if, on the Basic tab, the Decode Type is set to Words and Channels is set to either Fast Only or Both (Figure 7)
Within this dialog, note the four user-defined Payload Interpretation data fields labeled D0 through D3. These may be used to present the content of the message payload as it was programmed. Start by selecting the payload Offset, than specifying where it falls in the data field and how many nibbles is included.
For each of the four data bits (D0-D3) that are in use, specify the payload interpretation:
- Active: check to activate display of payload content
- Offset: Defines where content begins in the payload
- Nibbles: Number of nibbles included in the displayed value
- Order: Data may be displayed in least-significant-bit or most-significant-bit order
Teledyne LeCroy’s configurable SENT decoder is an extremely capable tool, enabling users to explore the intricacies of SENT signals as well as others encoded with the Manchester scheme. Armed with a bit of foreknowledge of the signal under test, users may exploit the decoder’s nearly unlimited flexibility with respect to signal parameters to cleanly decode and display signals and extract data relative to their physical characteristics.