CAN Bus Protocol — SMCS Kart

Status: v1 draft — source of truth. Any change to IDs or payloads must be made here first, then mirrored in firmware/common/kart_can.h.

Bus configuration

Property	Value
Bitrate	250 kbps (KART_CAN_BITRATE in firmware/common/kart_common/kart_can.h)
Termination	120 Ω at exactly two physical bus ends
Nodes	Teensy 4.1 (kart-core), Steervo ESP32
Traffic	11-bit standard IDs (this document)

Bitrate rationale. The Talon SRX is PWM-driven and off this bus, so the bitrate is ours to choose. Traffic is only two 50 Hz frames (~13 kbit/s), so we run a deliberately slow 250 kbps for noise margin on a bus whose differential termination is non-ideal — a longer bit time lets reflections settle before each sample point. Both nodes derive their timing from the single KART_CAN_BITRATE macro (Teensy FlexCAN::setBaudRate, ESP32 TWAI timing macro), so they cannot drift apart. Drop to 125000 for even more margin if the bench bus still logs errors; supported values are 1000000 / 500000 / 250000 / 125000.

The Talon SRX is not on the CAN bus: the Steervo drives it directly with a servo PWM signal (1.0 ms full reverse / 1.5 ms neutral / 2.0 ms full forward on Steervo GPIO25). So the only CAN traffic is the Teensy↔Steervo kart messages below. (Earlier drafts ran the Talon over CTRE 29-bit extended IDs on this same bus; that path is retired but the frame builders survive in firmware/steervo/lib/steervo/ctre_frames.h as a reference, ported from CanControl.) Keep custom IDs in the ranges below.

ID allocation

Range	Use
0x100–0x10F	Steering (Teensy ⇄ Steervo)
0x110–0x11F	Reserved: future RC/aux nodes
0x120–0x1FF	Reserved

All multi-byte fields are little-endian. Angles are centi-degrees (int16, +100 = 1.00° right of center).

Messages

0x100 STEER_SET — Teensy → Steervo, 50 Hz, DLC 4

Offset	Size	Field	Notes
0	1	flags	bit0 ENABLE (1 = motor may be energized). All other bits reserved, must be 0.
1	2	setpoint_cdeg (int16)	Target steering angle. Ignored when ENABLE=0.
3	1	seq (uint8)	Rolling counter, +1 per frame.

Staleness rule (safety-critical): if the Steervo receives no valid STEER_SET with ENABLE=1 for 150 ms, it de-energizes the motor, sets fault bit SETPOINT_STALE, and stays in READY until fresh frames resume.

0x101 STEER_STATUS — Steervo → Teensy, 50 Hz heartbeat, DLC 8

Offset	Size	Field	Notes
0	1	state	0 INIT · 1 READY · 2 ACTIVE · 3 FAULT · 4 CALIBRATING
1	1	fault_bits	bit0 POT_RANGE · bit1 POT_FROZEN · bit2 STALL · bit3 SETPOINT_STALE · bit4 TALON_LOST · bit5 NOT_CALIBRATED · bit6 OVER_TRAVEL (pot driven past a calibrated end stop — latched)
2	2	measured_cdeg (int16)	Current angle from the pot (calibrated).
4	1	output_pct (int8)	Motor command, −100…+100.
5	1	seq_echo (uint8)	seq of the last accepted STEER_SET.
6	2	pot_raw (uint16)	Raw filtered ADC value (diagnostics/calibration).

Heartbeat rule (safety-critical): the Teensy treats no STEER_STATUS for 150 ms — or any frame with state=FAULT — as a steering fault and performs a controlled stop.

0x102 STEER_CAL — Teensy → Steervo, on demand, DLC 2

Offset	Size	Field	Notes
0	1	cmd	0 ENTER · 1 MARK_CENTER · 2 MARK_LEFT · 3 MARK_RIGHT · 4 SAVE_EXIT · 5 ABORT
1	1	reserved (0)

Calibration commands are accepted only while the kart-core reports it is in SAFE (the Teensy must not send them otherwise). Results persist in ESP32 NVS.

0x103 STEER_CFG — Teensy → Steervo, bench only, DLC 8

Offset	Size	Field	Notes
0	1	param	1 KP · 2 KI · 3 KD · 4 OUTPUT_LIMIT_PCT · 5 SOFT_LIMIT_MARGIN_CDEG
1	4	value (float32 LE)
5	3	reserved (0)

Ignored unless Steervo state is READY or CALIBRATING (never ACTIVE).

Rates and timeouts summary

Link	Nominal rate	Timeout	On timeout
STEER_SET (Teensy→ESP32)	50 Hz	150 ms	Steervo: motor off + fault bit
STEER_STATUS (ESP32→Teensy)	50 Hz	150 ms	Teensy: controlled stop
Talon PWM (ESP32→Talon)	50 Hz	~100 ms (Talon-internal)	Talon self-neutralizes if pulses stop (hardware failsafe). Steervo also commands neutral (1.5 ms) whenever it is not ACTIVE.