Packet Protocol

The firmware communicates with the daemon over a fixed-length binary protocol. There are two packet types: the high-frequency ADC Data packet and the one-shot Settings Handshake.

---

Packet Type 1 — ADC Data

Emitted at: configured sample rate (default 100 Hz)
Length: 18 bytes

Structure

Offset	Bytes	Type	Field	Value
0	1	uint8	header_1	0xAA
1	1	uint8	header_2	0xBB
2	4	int32_le	ch0	Vertical (Z) — signed 24-bit, sign-extended
6	4	int32_le	ch1	North (N)
10	4	int32_le	ch2	East (E)
14	4	uint32_le	crc	CRC-32 of bytes 0–13

Python parsing (daemon)

import struct
from binascii import crc32

PACKET_FORMAT = "<BBiiiI"  # 18 bytes
PACKET_SIZE   = struct.calcsize(PACKET_FORMAT)  # → 18

header_1, header_2, ch0, ch1, ch2, rx_crc = struct.unpack(PACKET_FORMAT, data)

payload     = data[:-4]
calc_crc    = crc32(payload) & 0xFFFFFFFF
valid       = (calc_crc == rx_crc)

ADC value range

The ADS1256 outputs a signed 24-bit integer, sign-extended to 32 bits before packing:

Code	Value
0x007FFFFF	+8,388,607 (positive full-scale)
0x00000000	Zero (ground)
0xFF800000	−8,388,608 (negative full-scale)

---

Packet Type 2 — Settings Handshake

Emitted: once by the daemon on startup; echoed back by the MCU.
Length: 6 bytes

Structure

Offset	Bytes	Type	Field	Description
0	1	uint8	header_1	0xCC
1	1	uint8	header_2	0xDD
2	2	uint16_le	sampling_speed	Target output rate in Hz
4	1	uint8	adc_gain	PGA enum (0–6)
5	1	uint8	adc_data_rate	ADS1256 DRATE enum (0–15)

No checksum on the settings packet

The settings packet uses an XOR byte-checksum (daemon side legacy). The MCU verifies the echo by comparing the received 6 bytes byte-for-byte against what it sent — an implicit check that is sufficient for a one-shot startup exchange.

Handshake sequence

1. Daemon opens the serial port (this may reset AVR/STM32 boards via DTR).
2. Daemon waits 2 s for the MCU to finish booting and initialise the ADS1256 registers.
3. Daemon transmits the 6-byte MCUSettingsFrame.
4. MCU applies the settings and echoes the identical 6 bytes.
5. Daemon waits up to 10 s, scanning for the 0xCC 0xDD header to align the response.
6. If the echo matches → streaming starts.
   If no echo within 10 s → MCUNoResponse exception, daemon exits.

Python struct format

PACKET_FORMAT = "<BBHBB"  # 6 bytes
struct.pack(PACKET_FORMAT, 0xCC, 0xDD, sampling_rate, adc_gain, adc_data_rate)

---

Heartbeat

The daemon sends a single byte 0x01 every 500 ms to prove the link is alive. The MCU pauses streaming if no heartbeat arrives within ~1 s, preventing the UART TX buffer from overflowing into a dead host.

---

Buffer alignment

If the RS-422 link is noisy or the daemon restarts mid-packet, the receive buffer will be misaligned. The daemon uses a sliding-window strategy to re-sync:

while len(buffer) >= PACKET_SIZE:
    if buffer[0] == 0xAA and buffer[1] == 0xBB:
        process_packet(buffer[:PACKET_SIZE])
        del buffer[:PACKET_SIZE]
    else:
        del buffer[0]  # shift one byte and try again

A CRC failure also causes the daemon to discard only that packet and keep looking — it does not flush the whole buffer.

---

Baud rate & timing

At 250,000 bps (8-N-1) transmitting 18 bytes:

Stage	Time
Packet TX (144 bits)	0.576 ms
USB-Serial adapter latency	~0.2 ms
Daemon parse + queue	~0.1 ms
Total latency	~0.88 ms
Budget at 100 Hz	10 ms
Headroom	~9.1 ms