PIROM and Scratch EEPROM Supported SMBus Transactions

The PIROM responds to two SMBus packet types: Read Byte and Write Byte. However, since the PIROM is write-protected, it will acknowledge a Write Byte command but ignore the data. The Scratch EEPROM responds to Read Byte and Write Byte commands. Table: Read Byte SMBus Packet illustrates the Read Byte command. Table: Write Byte SMBus Packet illustrates the Write Byte command.

In the tables, ‘S’ represents a SMBus start bit, ‘P’ represents a stop bit, ‘A’ represents an acknowledge (ACK), and ‘///’ represents a negative acknowledge (NACK). The shaded bits are transmitted by the PIROM or Scratch EEPROM, and the bits that are not shaded are transmitted by the SMBus host controller. In the tables, the data addresses indicate eight bits.

The SMBus host controller should transmit eight bits with the most significant bit indicating which section of the EEPROM is to be addressed: the PIROM (MSB = 0) or the Scratch EEPROM (MSB = 1).