Share the packet format with the Pi instead of duplicating it

[taciturnaxolotl]

Why

This merges the definitions for the binary packed protocol so the rpi and the arduino don't go out of sync

Both ends now use a library from SensorHub

Depends on HEEV/SensorHub#2 to merge first. CI installs from SensorHub main

/* <sensorhub/sensorhub.h> — works everywhere including on arduino */
void    sh_parser_init  (sh_parser_t *parser);
bool    sh_parser_feed  (sh_parser_t *parser, uint8_t byte, sh_packet_t *out);   uint8_t sh_checksum     (const uint8_t *data, size_t length);
void    sh_encode_frame (const sh_packet_t *packet, uint8_t *buffer);

/* <sensorhub/serial.h> — POSIX only */
int  sh_serial_open        (const char *device);
void sh_serial_close       (int fd);
bool sh_serial_read_packet (int fd, sh_parser_t *parser, sh_packet_t *out);

etc

engine temp needs a defined address as currently its just reading zeros

[taciturnaxolotl]

Updated: the wire format now carries a version and a length byte from the start, rather than being retrofitted later.

The version byte exists for one specific failure. If the payload changes size, framing desyncs and you notice. But if it stays the same size and a field changes meaning, the checksum still passes and the receiver decodes confidently wrong data. That is exactly what happened on the Python side in April, when columns 5 and 6 silently went from Speed/button to voltage/timer_reset_button and nobody caught it for months.

The length byte lets an old receiver skip a packet from a newer sender and stay framed instead of desynchronising.

Payload also widened while it was cheap to do so, since breaking the format twice is much worse than once:

	before	now
temperatures	2 named floats	4 slots
analog	1	4 slots
digital	5 bytes, inputs only	8 inputs + 8 outputs, as bits
sequence	none	uint16_t

Outputs are reported now. The firmware drives a radiator fan and a water pump whose state appeared nowhere in telemetry, so there was no way to see or log what the car was doing to itself.

The sequence number makes dropped packets visible. A checksum cannot tell you about a packet that never arrived, so a link losing half its traffic looked identical to a healthy one.

Costs: frame 26 to 41 bytes, which takes the link from 4.5% to 7% utilisation, and the sketch from 18% to 19% of a Nano.

[PonderForge]

Version is interesting, yet kinda a waste to send each packet, same thing with packet length. If the packet structure version is the newest, then length and version wouldn't change at all. If anything, you may wanna add a interrupt on the nano when it receives a byte that causes it to hand over a version, then have the Sensor Hub throw an error if the version doesn't match. Add that and I'll approve this one, but I gotta finish looking over the SensorHub pull request.

Thanks for the effort btw!

[PonderForge]

Oh also, if you would mind checking to see if you can put the engine and radiator temp on a timer interrupt as well so that the packet sending wouldn't be bogged down by the temp polling. I'm thinking polling every 100ms, since the datasheet for these sensors says it only can really update every 90ish ms