SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The Process Call protocol consists of a Write Word message, followed by a Read Word message, without a stop condition between the two messages. A PEC byte can be appended to the read data from the slave as an option to the message protocol. The Master Control Register includes a PRC_CALL bit, which enables the transmission of a Process Call message onto the PMBus. The PMBus Interface will automatically generate a repeated start condition and initiate the Read Word portion of the message when the process call bit is enabled.
To complete the Write Word portion of the Process Call, the Transmit Data Register is loaded with the command byte in Bits 7-0 and the data bytes are loaded into Bits 23-8 of the register.
After programming the Master Control Register, the PMBus Interface initiates the Process Call Message on the PMBus. The firmware can wait for an End of Message interrupt from the interface to determine the validity of the message. Upon the receipt of the EOM, the Status Register should indicate the receipt of 2 bytes from the Read Word portion of the Process Call message and the status of the Slave acknowledgement of the transmit data. If PEC processing is enabled, the PEC_VAL bit within the Status Register indicates the accuracy of the PEC byte received from the Slave during the Read Word part of the message.
The PRC_CALL bit within the Master Control Register should be disabled on the next message not of the Process Call protocol. Please note that any write to the Master Control Register initiates a message, so reconfiguration of the Master is not recommended until the firmware requires a new message to be transmitted on the PMBus.