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SENTToUSB — STM32F042 USB ↔ SENT Bridge

A USB CDC (Virtual COM Port) adapter that decodes and encodes SAE J2716 SENT sensor signals. Connect any SENT sensor to a PC and receive decoded frames over a standard serial port using the SLCAN text protocol.

Full documentation and product page: https://ucandevices.github.io

Table of Contents

  1. What it does
  2. Hardware
  3. Pinout
  4. Getting Started
  5. SLCAN Protocol Reference
  6. CAN Frame Reference
  7. Python Tools
  8. Sensor Presets
  9. Learn Mode
  10. TX Mode
  11. Firmware Defaults
  12. Quick Reference Card

What it does

  • Receives SENT signals from automotive sensors (SAE J2716 / ISO 21097)
  • Decodes each frame: sync detection, nibble extraction, CRC validation
  • Sends decoded data to the PC as SLCAN t-frames over USB CDC
  • Transmits SENT frames synthesised from host-supplied CAN data (TX mode)
  • Auto-detects tick period, nibble count and CRC mode via Learn mode
  • Supports 4-, 6- and 8-nibble sensors, both CRC modes, seeds 0x03 and 0x05
  • USB-C connector, no driver needed on Windows 10+, Linux, macOS

Hardware

Item Value
MCU STM32F042K6
Clock 48 MHz HSI48 (no external crystal)
USB Full-Speed 12 Mbit/s, CDC class (Virtual COM Port)
SENT RX timer TIM2 CH3 — 48 MHz input capture, 16-bit counter
SENT TX timer TIM14 — prescaler 143 → 1 tick = 3 µs
Connector USB-C
Size 17 × 28 mm (without USB connector)
Bootloader USB DFU compatible

Pinout

Pin Direction Function
PA2 Input SENT RX — TIM2 CH3 rising-edge capture
PA1 Output SENT TX — TIM14-driven push-pull
  • RX pull-up: fitted on the board by default — no external pull-up needed.
  • TX pull-up: optional, enable by closing the onboard solder jumper.

Getting Started

  1. Plug the device into a USB port.
  2. The OS creates a Virtual COM Port (COM3 on Windows, /dev/ttyACM0 on Linux).
  3. Open the port at any baud rate — USB CDC ignores baud rate settings.
  4. Send O\r to start receiving SENT data.

Minimal session:

→  O\r                              open channel / start RX with default config
←  \r                               ACK

→  t001706000305050510\r            configure: 6 nibbles, DATA_ONLY, seed=0x03,
←  z\r                                  tick 2.5–5 µs, output CAN ID=0x510

←  t5103AABBCC\r                    decoded SENT frame (repeats ~100–1000×/s)

→  C\r                              stop
←  \r

SLCAN Protocol Reference

All commands are plain ASCII terminated with \r (CR, 0x0D). Responses end with \r; the NACK is a bare \a (BEL, 0x07).

Commands

Command Description Response
O\r Open channel — starts SENT RX \r
L\r Listen-only open — same as O \r
C\r Close channel — stops all activity \r
V\r Hardware version query V0101\r
v\r Firmware version query v0101\r
N\r Serial number query N + 4 hex digits unique to each MCU
F\r Status flags F00\r
t<ID><DLC><DATA>\r Send a CAN frame (control / config / TX data) z\r or \a

The serial number returned by N is a 16-bit XOR-fold of the STM32F042 96-bit unique device ID, so each unit reports a different value.

Responses

Response Meaning
\r Command ACK
\a NACK — malformed command or invalid config
z\r Frame ACK (reply to a t command)
t<ID><DLC><DATA>\r Decoded SENT frame pushed to host (unsolicited)

CAN Frame Reference

All frames use standard 11-bit IDs with the t prefix.

Host → Device

0x001 — Configuration (DLC = 7)

Byte Field Notes
B0 Data nibbles 4, 6, or 8
B1 CRC mode 0 = DATA_ONLY, 1 = STATUS_AND_DATA
B2 CRC init seed 0x03 (SAE APR2016) or 0x05 (legacy/GM)
B3 Min tick units of 0.5 µs (e.g. 5 → 2.5 µs)
B4 Max tick units of 1 µs (e.g. 5 → 5.0 µs)
B5–B6 Output RX CAN ID big-endian, 11-bit (e.g. 0x510 → 05 10)

Example — 6 nibbles, DATA_ONLY, seed=0x03, tick 2.5–5 µs, output ID=0x510:

t001706000305050510\r

0x600 — Control (DLC = 1)

Byte Action
01 Start RX — begin receiving and decoding SENT frames
02 Start TX — begin transmitting frames supplied via 0x520
03 Stop — halt all activity
04 Learn — auto-detect tick, nibble count and CRC mode 
t600101\r    start RX
t600102\r    start TX
t600103\r    stop
t600104\r    start learn mode

Learn mode: device replies z\r immediately, streams normal decoded frames while searching, then sends one 0x601 result frame and stops RX — send O\r or t600101\r to resume.

0x520 — TX Frame Data (DLC ≥ 5, TX mode only)

Byte Field
B0 Status nibble (low 4 bits)
B1–B4 Data nibbles packed big-endian (nibble[0] at bits 31..28)
B5–B6 Pause ticks, little-endian (optional, default 12)

Example — status=1, nibbles [1,2,3,4,5,6] → packed 0x00123456:

t52050100123456\r

Device → Host

Configured ID (default 0x510) — Decoded RX Frame

Pushed for every CRC-valid decoded SENT frame. Two nibbles packed per byte (high nibble first).

t5103AABBCC\r     3 bytes for a 6-nibble sensor

Unpack: nibble[0] = byte[0] >> 4, nibble[1] = byte[0] & 0x0F, etc.

The output CAN ID is set via B5–B6 of the 0x001 config frame (default 0x510).

0x601 — Learn Result (DLC = 4)

Sent once when learn mode locks.

Byte Field
B0–B1 Learned tick × 10 in units of 0.1 µs, little-endian (e.g. 1E 00 = 30 → 3.0 µs)
B2 Nibble count locked: 4, 6, or 8
B3 CRC mode: 0 = DATA_ONLY, 1 = STATUS_AND_DATA

Python Tools

Requirements

pip install pyserial

Python 3.10+ (tkinter included with most Python distributions).

sent_viewer.py — GUI Monitor

Real-time GUI for monitoring and transmitting SENT frames.

python sent_viewer.py

Left panel — controls:

Control Description
Port selector Lists available COM ports with auto-refresh
Connect / Disconnect Opens port, sends O\r; close sends C\r
Preset Loads all parameters for MLX90377, 04L 906 051 L, or GM 12643955
Tick Size (µs) Sensor tick reference; used to compute min/max tick for config frame
Data Nibbles 4, 6, or 8
CRC Seed 0x03 (SAE APR2016) or 0x05 (legacy/GM)
CRC Mode DATA_ONLY or STATUS_AND_DATA
RX CAN ID Output CAN ID of decoded frames (default 0x510)
Apply Config Sends current parameters as a 0x001 config frame
Learn Sends t600104\r; updates fields automatically from the 0x601 result
Pause / Clear Freeze display or reset counters and table

Right panel — display:

  • Large blue hex bytes — last decoded frame data
  • Nibble row — individual nibbles unpacked
  • Stats bar — frame count, CRC errors, sync errors, rate (fps)
  • Received Frames table — scrollable history with timestamp, CAN ID, DLC, data, nibbles
  • Raw Log — colour-coded SLCAN traffic

TX SENT Frame panel (right of the live display):

Field Description
Status 1 hex nibble (status field of the SENT frame)
Nibbles Hex string of data nibbles (e.g. 123456 for 6 nibbles)
Period (ms) Repeat interval; minimum 10 ms
Send One-shot: sends one frame then returns to RX mode
Start / Stop Periodic: switches to TX mode, sends at the set period; Stop resumes RX

sent_test.py — CLI Test Tool

Command-line tool for quick validation without the GUI.

python sent_test.py COM8
python sent_test.py COM8 --nibbles 6 --crc-mode 0 --seed 0x03 --can-id 0x510
python sent_test.py COM8 --learn

Options:

Flag Default Description
port (required) Serial port, e.g. COM8
--nibbles 6 Data nibbles: 4, 6, or 8
--crc-mode 0 0 = DATA_ONLY, 1 = STATUS_AND_DATA
--seed 0x03 CRC init seed
--tick-min 5 Min tick in units of 0.5 µs
--tick-max 5 Max tick in units of 1 µs
--can-id 0x510 Output RX CAN ID
--learn off Run learn mode instead of fixed config
--timeout 10.0 Listen duration in seconds

The tool queries V, v, N at startup, sends the config or learn command, then prints all received frames for the timeout duration.

Sensor Presets

Sensor Tick Nibbles CRC Mode Seed
MLX90377 (angle) 3 µs 6 DATA_ONLY 0x03
04L 906 051 L (VW/Audi DPF) ~3 µs 6 STATUS_AND_DATA 0x05
GM 12643955 (MAP) ~3 µs 6 DATA_ONLY 0x05

MLX90377 nibble decode

Frame bytes: A5 3C 7F
Nibbles:     A  5  3  C  7  F

angle = (A<<8 | 5<<4 | 3) = 0xA53
mag   = (C<<8 | 7<<4 | F) = 0xC7F

Learn Mode

When the sensor type or tick period is unknown:

t600104\r

The firmware tries all combinations of {4, 6, 8} nibbles × {DATA_ONLY, STATUS_AND_DATA} CRC modes. It requires 3 consecutive CRC-valid frames before locking. Once locked it sends one 0x601 result frame and stops RX.

Resume after learning:

O\r          (or t600101\r)

The sent_viewer.py Learn button handles this automatically — it sends the learn command, parses the 0x601 result, and populates the parameter fields.

TX Mode

The device synthesises and transmits SENT frames on PA1.

t600102\r              start TX mode
t52050100123456\r      transmit: status=1, nibbles [1,2,3,4,5,6]
t600101\r              switch back to RX mode

TX timing: TIM14 at 48 MHz, prescaler 143 → 1 tick = 3 µs. Frame sequence: sync (56 ticks) + status nibble + N data nibbles + CRC nibble + pause (default 12 ticks).

The TX SENT Frame panel in sent_viewer.py automates mode switching:

  • Send fires a single frame and returns to RX
  • Start/Stop sends frames at the configured period (≥ 10 ms) until stopped

Firmware Defaults

Parameter Default
SENT tick 3 µs (min accepted 2.5 µs)
Data nibbles 6
CRC mode DATA_ONLY
CRC seed 0x03 (SAE APR2016)
Output CAN ID 0x510
TX pause ticks 12
RX active at power-on No — starts after O\r or t600101\r

Quick Reference Card

OPEN / START RX        O\r
CLOSE / STOP           C\r
VERSION                V\r  ->  V0101\r
FIRMWARE               v\r  ->  v0101\r
SERIAL NUMBER          N\r  ->  Nxxxx\r  (unique per device)
STATUS FLAGS           F\r  ->  F00\r

START RX  (CAN ctrl)   t600101\r
START TX  (CAN ctrl)   t600102\r
STOP      (CAN ctrl)   t600103\r
LEARN     (CAN ctrl)   t600104\r

CONFIG (6-nibble, DATA_ONLY, seed=0x03, tick 2.5-5us, ID=0x510)
                       t001706000305050510\r

TX FRAME (status=1, nibbles 1,2,3,4,5,6)
                       t52050100123456\r

DECODED DATA OUT       t5103AABBCC\r   (ID=0x510, DLC=3)
LEARN RESULT           t60104LLHHNCM\r (ID=0x601, DLC=4)

ACK                    \r
NACK                   \a  (0x07)
FRAME ACK              z\r

Full documentation: https://ucandevices.github.io

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SAE J2716 SENT (Single Edge Nibble Transmission) to USB converter

ucandevices.github.io/sentusb.html

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protocol sensors automotive sent j2716

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