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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.
10.7.12 Group Command
The PMBus Interface must support the Group Command Protocol. The Group Command Protocol is used
to send commands to more than one device within the same message. When devices on the PMBus
detect the stop condition at the conclusion of the Group Command message, the received commands are
executed concurrently. To initiate a Group Command, the GRP_CMD bit within the Master Control
Register must be set when programming the slave address for the first device in the message. The rest of
the message is processed as a write byte/word message. At the conclusion of the first part of the Group
Command message, the firmware programs the next device address in the Master Control Register. The
PMBus Interface will initiate a repeated start on the bus and start the next part of the message. When
programming the last device address of the Group Command message, the firmware must disable the
GRP CMD bit when programming the Master Control Register.
10.8 PMBUS Communications Fault Handling
The UCD3138 PMBus interface hardware and firmware is designed to handle communications faults
caused by glitches, interruptions, and stuck signals on the PMBus lines. In many cases, the system will
recover immediately. In some cases, for example if one or both of the lines are stuck high or low, no
communication will be possible. There are a few cases in which the system will may require an additional
message transmission for recovery. Here are two of them:
10.8.1 Bit Counter
If a PMBus message to the UCD is halted in the middle of a byte, the UCD bit counter may not be reset to
zero. Therefore the UCD may not respond correctly to the next message. Normally the bit counter will be
resychronized after this one incorrect response, so subsequent messages will be handled correctly.
On the UCD3138, and UCD3138064, this is the case. On the UCD3138A64 and UCD3138128 and all A
versions, stopping the message in the middle of the byte should be handled correctly.
SNIU028A – February 2016 – Revised April 2016
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Figure 10-52.
Copyright © 2016, Texas Instruments Incorporated
Master Mode Operation Reference
PMBus Interface/I2C Interface
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