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I2C - Frame Format

I2C Communication Overview

I2C communication involves two roles: a master and a slave. The master initiates and controls the communication, while the slave responds to the master's requests. Data transmission on the I2C bus is organized into transactions, each starting with a START condition and ending with a STOP condition. Each transaction begins with a 7-bit address and comprises one or more data bytes, with each byte followed by an acknowledgment from the receiver.

I2C Frame Evaluation

I2C communication is captured as frames by a built-in low-level I2C analyzer. Each frame represents one byte of information. Frame types are defined below including: start, address, data, and stop.

Data Frame Evaluation

The HLA executes once for each incoming frame. Each frame contains only one byte. Each transaction begins with a start frame, an address frame, and then any number of data frames, ending with a stop frame. A state machine is often needed to decode multi-byte (multi-frame) sequences such as a command followed by a parameter.

Output Frame Format

Frame Type: "address"

  • I2C address byte
Property Type Description
address byte The 7 bit I2C address
read bool True for read operations, false for write operations
ack bool True when the address was ACKed, false when NAKed
error str (optional) Present if an there was a problem reading the I2C data

Example

Store the address as an integer and display it on the terminal as a hex value

if frame.type == 'address':
  address = frame.data['address'][0]
  print(f"Address: {hex(address)}")

Result

Data: 0x3C

Frame Type: "data"

  • I2C data byte
Property Type Description
data byte single 8 bit data word
ack bool True when the data byte was ACKed, false when NAKed
error str (optional) Present if an there was a problem reading the I2C data

Example

Store the data value as an integer and display it on the terminal as a hex value

if frame.type == 'data':
  data = frame.data['data'][0]
  print(f"Data: {hex(data)}")

Result:

Data: 0x00

Frame Type: "start"

  • undefined
  • I2C start condition
Property Type Description

Frame Type: "stop"

  • undefined
  • I2C stop condition
Property Type Description