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SENT

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SENT (SAE J2716)

Overview

SENT (Single Edge Nibble Transmission) is a simple one-way serial protocol for high-speed sensor data in automotive applications. It transmits 4-bit nibbles using pulse-width encoding on a single wire, used in pedal position, pressure, and temperature sensors.

1. Theory & Fundamentals

  • One-wire unidirectional: sensor to ECU only
  • Voltage: 5V supply; signal swings 0–5V
  • Data: encoded as pulse widths (1 nibble per pulse)
  • Tick time: configurable 3–90 µs; nominal 3 µs typical
  • Nibble: 12–27 ticks wide; value = (width/tick)−12 → 0–15
  • Frame: Sync pulse + 1-6 data nibbles + optional CRC nibble + pause
  • Speed: ~20 kbps practical; clock-free (self-clocking)

2. Frame / Packet Structure

SENT frame:
  SYNC (≥56 ticks) | STATUS(4b) | D1(4b) | D2(4b) | ... | CRC(4b) | PAUSE

Nibble timing:
  Falling edge starts pulse; next falling edge ends it
  Width in ticks: 12=0, 13=1, ..., 27=15 (16 values = 4 bits)

SlowChannel (serial data in status nibble):
  Short serial: 2 IDs × 8 data bits multiplexed over 16 fast frames
  Enhanced serial: 16-bit message in status nibble sequence

Fast channel data example (throttle position):
  STATUS | D1(MSB) | D2 | D3 | D4(LSB) | CRC → 16-bit position + 4-bit status

3. Protocol Mechanics

  • Asynchronous from ECU perspective: ECU timestamps falling edges via timer capture
  • Tick calibration: Measured from SYNC pulse length
  • CRC: nibble-level CRC (recommended nibble CRC by SAE J2716)
  • Slow channel: encodes diagnostic/ID data in status nibble bit patterns
  • Fault: SYNC missing or timing outside spec flags sensor fail

4. Hardware Implementation

  • Single GPIO input configured as input capture timer
  • 5V supply to sensor; SENT signal direct to 5V-tolerant MCU or via level shifter
  • Pull-up: not needed (SENT is push-pull)
  • MCU: STM32 TIM input capture, PIC CCP module
  • Common sensors: TDK MAP sensor, Sensata pressure sensor

5. Register-Level / Configuration

// STM32 TIM2 input capture for SENT decoding
void TIM2_IRQHandler(void) {
    static uint32_t last_cap = 0;
    uint32_t cap = TIM2->CCR1;
    uint32_t width = cap - last_cap; last_cap = cap;
    // Convert width to tick count
    uint32_t ticks = width / tick_time_us;
    if(ticks >= 56) { // SYNC detected
        nibble_index = 0;
    } else if(ticks >= 12 && ticks <= 27) {
        nibbles[nibble_index++] = ticks - 12; // 0–15
    }
    if(nibble_index >= 6) process_frame(nibbles);
}

6. Driver / Software Development

  • Implement init, TX, RX functions; use interrupts or DMA
  • Handle errors: timeout, CRC mismatch, arbitration loss
  • Use circular/ring buffers for high-throughput RX
  • Add retry logic and watchdog for reliability
  • Separate hardware layer from protocol logic

7. Debugging & Testing

  • Logic analyzer: measure pulse widths, verify nibble values
  • Oscilloscope: check SYNC pulse ≥56 ticks; verify signal swing 0–5V
  • Common issues: tick time calibration off; CRC mismatch; slow-channel decode wrong
  • Compare decoded nibbles vs expected sensor output range

8. Real-World Applications

  1. Throttle pedal position sensors
  2. Transmission oil pressure sensors
  3. EGR valve position
  4. Turbocharger boost pressure
  5. Temperature sensors

9. Advanced Topics & Edge Cases

  • SENT short serial vs enhanced serial: different slow-channel encoding
  • Multi-wire SENT: Some ECUs accept 2 SENT sensors (A and B redundancy)
  • OEM variants: Some add proprietary diagnostic nibbles
  • SENT vs SPI: SENT simpler wiring; SPI higher speed
  • Transition to PSI5/SENT: Both used for modern sensor interfaces

10. Standards & Variants

Parameter Typical value
Tick time 3 µs
SYNC pulse ≥56 ticks (168 µs)
Nibble range 12–27 ticks
Frame rate up to 1 kHz
Wires 3 (VCC, GND, SENT)

💡 Practical Examples

Example 1

Calibrate tick: measure SYNC pulse width in µs, divide by 56 to get tick_time

Example 2

Decode pressure: D1–D3 nibbles → 12-bit value → apply sensor scaling formula

Example 3

CRC check: compute nibble CRC over status+data nibbles, compare with received CRC nibble

🧪 Practice Questions

Beginner

  1. How many bits does each SENT nibble carry?
  2. What is the SYNC pulse width?
  3. Is SENT bidirectional?
  4. What is a tick in SENT?
  5. What does the CRC nibble protect?

    6. Intermediate

  6. Implement SENT decoder using STM32 input capture timer.
  7. Calculate nibble value from measured pulse width.
  8. Implement SENT slow-channel short serial decoding.
  9. How do you calibrate tick time from the SYNC pulse?
  10. What causes CRC errors in SENT?

    11. Advanced

  11. Decode all nibbles, apply scaling to get throttle % and temperature.
  12. Add slow-channel enhanced serial decoder for sensor diagnostics.
  13. Implement redundant SENT receiver comparing two sensors.
  14. Build SENT simulator transmitting test patterns on GPIO.
  15. Add fault detection: missing SYNC, out-of-range nibbles.

    16. Hands-on Projects

  16. SENT Decoder: input capture → nibble decode → scaled value on display.
  17. Sensor Simulator: generate SENT waveform on GPIO for testing ECU.
  18. Logger: capture 1 kHz SENT stream, log to CSV.

Checklist

  • [ ] Understand SENT pulse-width encoding
  • [ ] Configure TIM input capture for edge detection
  • [ ] Detect SYNC pulse
  • [ ] Decode data nibbles from pulse widths
  • [ ] Validate CRC nibble
  • [ ] Implement slow-channel short serial decode
  • [ ] Calibrate tick time from SYNC
  • [ ] Apply sensor scaling formula
  • [ ] Debug with logic analyzer
  • [ ] Handle SENT fault conditions
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