SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The PMBus Interface has the capability to initiate any of the available message protocols used for communication with connected slave modules. Initiating a message begins with programming the Master Control Register (Address 0h). Upon programming of this register, the message will begin transmission on the PMBus once the bus is idle and ready for additional messages.
Note that the default mode of the PMBus interface is Slave mode. To operate in Master Mode, it is necessary to clear the SLAVE_EN bit and to set the MASTER_EN bit in the PMBCTRL3 register.
Within the bits of the Master Control Register, a number of options are provided that help to configure the PMBus message. The PMBus Interface includes an optional PEC enable bit (Bit 18). Enabled the PEC_EN bit forces the PMBus Interface to append a PEC byte onto the end of the message. The firmware is not required to calculate the PEC value or account for the PEC byte when entering the number of bytes in the message.
The Byte Count bits (Bits 15-8) within the Master Control Register configures the number of data bytes within the outgoing message. The firmware is required to program the byte count at the start of each message. A byte count of zero will result in the transmission of a Quick Command message. The byte count does not include any command bytes, PEC bytes, address bytes or the block length (found in Block Write/Read messages). The PMBus Interface in Master Mode automatically terminates a valid message based on the values programmed in the byte count bits. In cases of a slave NACK, the PMBus Interface also automatically initiates a stop condition to terminate the message and provides the appropriate alarms to the firmware through the Status Register.
Inclusion of command bytes in the message initiated by the Master is configured through the CMD_EN and EXT_CMD bits (Bits 17-16 of the Master Control Register). Enabling CMD_EN forces the PMBus Interface to include a single command byte in the message. On the first programming of the Transmit Data Register, bits 7-0 will represent the command byte. When enabling EXT_CMD, support for extended commands is enabled. Bits 15-0 of the Transmit Data Register, after the first program of the Transmit Data Register, represent the extended command bytes.
Additional control bits within the Master Control Register enable various message protocols for PMBus applications. Protocols such as Process Call and Group Command Messages require additional programming with these bits.