SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The PMBus Interface provides support for extended commands which allow for an extra 256 command codes. By asserting the EXT_CMD bit within the Master Control Register, two command bytes are transmitted on the message protocol. Extended commands can be added to the Read Byte, Read Word, Write Byte and Write Word protocols. Operation of the PMBus interface in extended command mode is similar to these formats. In programming the write data or first part of the read message, the second command byte is loaded into Bits 15-8 of the Transmit Data Register with the remaining data bytes. The remaining operation of the PMBus is identical to the previous protocols, except for the inclusion of a Repeated Start condition and slave address in the write messages. No support is required by firmware for these additional bytes in the write messages. The interface will interpret the EXT_CMD bit and make the appropriate format changes.