Protocol Specification

Message Format (Variable Length: 4–31 bytes) 

┌──────────┬─────────────┬──────────┬─────────────────┬──────────┐
│  typeID  │ commandType │ data_len │   data[0..N]    │   crc8   │
│ (1 byte) │  (1 byte)   │ (1 byte) │ (0–27 bytes)    │ (1 byte) │
└──────────┴─────────────┴──────────┴─────────────────┴──────────┘
Field Size Description
typeID 1 byte Module type (sensor=1, motor=2, etc.)
commandType 1 byte Command (read=0, set=1, reset=2, etc.)
data_len 1 byte Number of payload bytes (0–27)
data[] 0–27 bytes Opaque payload data (user-defined format)
crc8 1 byte CRC-8 over typeID, commandType, data_len, data[]

Notes:

  • address exists in the struct but is not serialized (used for routing at the application layer).
  • Maximum frame size is 31 bytes to fit within Arduino Wire’s 32-byte buffer.
  • CRC excludes address and the CRC byte itself.
  • Payload is opaque bytes; applications can encode floats, ints, structs, etc.

Communication Patterns 

Controller [4–31 byte message]> Peripheral    # Send command
Controller [I2C request]> Peripheral          # Request data
Controller <[4–31 byte response] Peripheral   # Response

Standard Values

Module Types

  • 0: Generic
  • 1: Sensor
  • 2: Actuator
  • 3: Display
  • 4: Input

Command Types

  • 0: Read data
  • 1: Set parameters
  • 2: Reset
  • 3: Calibrate

CRC

  • Polynomial: 0x07 (CRC-8)
  • Calculated using crc8_nibble_calculate() (crc8_nibble variant)
  • Applied to serialized header + payload (bytes 0 through 2 + data_len)

Timing Guidelines

  • 10ms minimum between messages
  • 50ms timeout for peripheral response
  • 100kHz I2C clock speed (standard)
  • Addresses 0x08-0x77 usable

Example: Encoding a Float

To send a float value in the payload:

crumbs_message_t m = {0};
m.type_id = 1;
m.command_type = 1;

float value = 3.14f;
m.data_len = sizeof(float);
memcpy(m.data, &value, sizeof(float));