PMBus Example

The following diagram illustrates a typical PMBus command.

Figure 1. Two Byte PMBUS Write Format

Figure 2. Two Byte PMBUS READ Format

Table 1. SYSMON PMBus Label Descriptions
Command	Description
S_M	Start command - master to slave
A_M[6:0]	7-bit I2C slave address - master to slave
R/W_M	Read (1) / Write (0) command - master to slave
ACK_S	Acknowledge - slave to master
CMD_M[7:0]	PMBUS Command
D_M[7:0],D_M[15:8],D_M[23:16],D_M[31:24]	Data Bytes sent by master with the high bit sent firstwith theleast significant byte sent first.
Sr_M	Repeated start command - master to slave
D_S[7:0],D_S[15:8],D_S[23:16],D_S[31:24]	Data Bytes sent by slave with the high bit sent first with least significant byte sent first.
ACK_M	Acknowledge - master to slave
NACK_M	Not acknowledge - master to slave
P_M	Stop command - master to slave

Many PMBus commands require multiple byte read and write commands. The following diagram illustrates a general overview of the various sized commands supported by SYSMON.

Figure 3. Command Sequences

Using MFR_SELECT_REG and MFR_ACCESS_REG

When in De-restricted mode, PMBUS can be used to directly access the SYSMON register space through the MFR_SELECT_REG(MFR_SPECIFIC_D0) and MFR_ACCESS_REG(MFR_SPECIFIC_D1) commands. MFR_SELECT_REG is a two byte command that selects the SYSMON Register to be accessed with the four byte MFR_ACCESS_REG command. The MFR_ACCESS_REG can read SYSMON registers as well as write to unrestricted bits when the LOCK (PCSR_LOCK) has been unlocked. The two-byte MFR_SELECT_COMMAND is set by writing the 16 LSBs of the absolute address of the SYSMON register space, for example to access the REG_PCSR_LOCK register (0x00F127000C), 0x000C is written a two byte write with the command 0XD0. Once the Register has been defined, a 4 byte READ or WRITE command can be used to modify non-restricted bits to the register defined from the MFR_SELECT_REG command.