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IO-Jack — I²C Read & Write

How It Works

In I²C mode, the IO jack becomes an I²C master. Read/write transfers and setting/reading the I²C address are carried out via a single command byte, followed by the actual byte stream.

This byte is composite: bit 7 = R/W marker (0 = read, 1 = write), bits 6–0 = length (1–127 = number of data bytes; 0 = address operation):

Bit position 7 6 5 4 3 2 1 0
Field R/W L6 L5 L4 L3 L2 L1 L0
Marker Length (0–127) Length Length Length Length Length Length

This gives four operations – command byte plus data bytes as applicable:

Operation 7 6 5 4 3 2 1 0 = Byte
Read I²C (n bytes) 0 L6 L5 L4 L3 L2 L1 L0 0nnn nnnn (n = 1–127)
Write I²C (n bytes) 1 L6 L5 L4 L3 L2 L1 L0 1nnn nnnn (= 128 + n)
Read I²C Address 0 0 0 0 0 0 0 0 0x00
Write I²C Address 1 0 0 0 0 0 0 0 0x80

The subsequent byte stream and the response:

Operation Further bytes Response
Read I²C – n data bytes (byte 1 … byte n)
Write I²C n data bytes n bytes written
Read I²C Address – current I²C address
Write I²C Address Address byte Confirmation

Example values: 0x00 = read address, 0x80 = write address, 0x01 = read with length 1 → one byte written.

Protocol Details

Read I²C

Request:   Length  [1–127]
Response:  Byte₁  Byte₂  …  Byteₙ         (n = Length)

Write I²C

Request:   (128 + Length) [129–255]   Byte₁  Byte₂  …  Byteₙ
Response:  "n bytes written"

Address Operations

Action Request Response
Read I²C Address 0x00 I²C address
Write I²C Address 0x80 (128) Confirmation

Cross References