Add Matek Serialshot protocol

[digitalentity]

Initial cut on MATEK's SerialShort protocol.
No telemetry (we don't support ESC telemetry yet).
Not tested. May break DSHOT.

[digitalentity]

@MATEKSYS FYI

[MATEKSYS]

Thank you. we will test it and try supporting telemetry with a workaround

[digitalentity]

@MATEKSYS I would recommend doing some changes to the protocol to make it more robust. I'll publish a proposed throttle packet format soon.

I'm also happy to make tweaks to your ESC code if necessary.

[digitalentity]

Suggested packet format:

typedef struct __attribute__((packed)) {
    uint8_t hdr;			// Header/version marker
    uint8_t motorData[6];	// 12 bit per motor
    uint8_t crc;			// CRC8/DVB-T of hdr & motorData
} serialShortPacket_t;

// Motor throttle values encoding
motorData[0] = motor[0] & 0x00FF;
motorData[1] = motor[1] & 0x00FF;
motorData[2] = motor[2] & 0x00FF;
motorData[3] = motor[3] & 0x00FF;
motorData[4] = (((motor[0] & 0xF00) >> 8) << 0) | (((motor[1] & 0xF00) >> 8) << 4);
motorData[5] = (((motor[2] & 0xF00) >> 8) << 0) | (((motor[3] & 0xF00) >> 8) << 4);

// Motor throttle value decoding
motor[0] = motorData[0] | (((uint16_t)((motorData[4] >> 0) & 0x0F)) << 8);
motor[1] = motorData[1] | (((uint16_t)((motorData[4] >> 4) & 0x0F)) << 8);
motor[2] = motorData[2] | (((uint16_t)((motorData[4] >> 0) & 0x0F)) << 8);
motor[3] = motorData[3] | (((uint16_t)((motorData[4] >> 4) & 0x0F)) << 8);

Pros:

• Packet size of 8 byte (same as now).
• 12 bit throttle resolution (0...4096) which is plenty to accommodate raw DSHOT throttle values/commands.
• Extendability - we can have different header values to indicate different type of packet data (i.e. motors 5-8, or servo data).
• Robust. Even if we have bit errors - CRC will detect them. Also, CRC is more robust compared to simple addition.

Cons:

• Slightly higher encoding/decoding overhead due to bit manipulation and CRC calculation. Shouldn't be an issue.

[digitalentity]

@MATEKSYS what do you think about changing the ESC protocol to make it more error-immune?

[MATEKSYS]

We have modified the protocol according to your suggestion. need to test it.

[digitalentity]

@MATEKSYS this is super cool!!! Thanks!

[digitalentity]

@MATEKSYS see the last commit for reference implementation of new throttle packet format.
Verified to generate correct DSHOT throttle values on all 4 motors.

Also, I missed the fact that new on-wire packet is 1 byte smaller (just 8 bytes now instead of 9)

[digitalentity]

Verified to be working. Guidelines for setup for MATEKF405 board with ESC connected to UART1

serial 0 262144 115200 115200 0 115200
set min_throttle = 1050
set max_throttle = 2000
set motor_pwm_rate = 2000
set motor_pwm_protocol = SERIALSHOT

[digitalentity]

Checked that DSHOT still works. Ready to merge.