SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The Read Byte and Read Word messages consist of a device address, a command byte, received data bytes from a slave and an optional PEC byte. Read Byte messages include a single byte, while the Read Word message protocol supports receipt of 2 bytes from the slave. Similar to the Receive Byte Protocol, the Master Control Register is configured to receive 1 or 2 bytes, the CMD_EN bit is set and the PEC_EN is configured to expect or not expect a PEC byte appended to the message. The PMBus Interface will automatically terminate the message after the expected number of bytes is received from the slave or if the slave does not properly acknowledge any portion of the message.
In addition to programming the Master Control Register, the firmware is expected to load the command byte into bits 7-0 of the Transmit Data Register. Any data received from the slave will be found in the Receive Data Register.
After programming the Master Control Register, the PMBus Interface initiates a Read Byte/Read Word message on the PMBus. The firmware can wait for an End of Message interrupt from the interface. Upon the EOM interrupt, the Status Register is read to determine the number of bytes received (1 for a Read Byte/2 for a Read Word). The received data bytes are found in Bits 15-0 of the Receive Data Register. If PEC is enabled for the message, the PEC_VAL bit in the Status Register can be verified to check the accuracy of the received PEC byte from the Slave.