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
PropertyTypeDescription
addressbyteThe 7 bit I2C address
readboolTrue for read operations, false for write operations
ackboolTrue when the address was ACKed, false when NAKed
errorstr(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
PropertyTypeDescription
databytesingle 8 bit data word
ackboolTrue when the data byte was ACKed, false when NAKed
errorstr(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

Frame Type: "stop"

  • undefined
  • I2C stop condition