Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 4/01/2024
Public

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5.9.6.3.4. START BYTE Transfer Protocol

The START BYTE transfer protocol is set up for systems that do not have an on-board dedicated I2C hardware module. When the I2C controller is set as a slave, it always samples the I2C bus at the highest speed supported so that it never requires a START BYTE transfer. However, when I2C controller is set as a master, it supports the generation of START BYTE transfers at the beginning of every transfer in case a slave device requires it. This protocol consists of seven zeros being transmitted followed by a 1, as illustrated in the following figure. This allows the processor that is polling the bus to under-sample the address phase until the microcontroller detects a 0. Once the microcontroller detects a 0, it switches from the under sampling rate to the correct rate of the master.

Figure 215. START BYTE Transfer

The START BYTE has the following procedure:

  1. Master generates a START condition.
  2. Master transmits the START byte (0000 0001).
  3. Master transmits the ACK clock pulse. (Present only to conform with the byte handling format used on the bus)
  4. No slave sets the ACK signal to 0.
  5. Master generates a RESTART (R) condition.

A hardware receiver does not respond to the START BYTE because it is a reserved address and resets after the RESTART condition is generated.