ADM1266ACPZ-R7 datasheet(31/62 Pages) AD

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Data Sheet

ADM1266

Rev. A | Page 31 of 62

GROUP COMMAND PROTOCOL

In addition to the communication protocols described in the

Data Transfer Commands section, the PMBus slave supports

a special group command in which commands are sent to

multiple slaves in a single serial transmission. The commands

to each slave can be different from one another, with each set

of slave address and command separated by a repeated start (Sr)

bit (see Figure 28). At the end of a transmission to all slaves, a

single stop (P) bit is sent to initiate concurrent execution of the

received commands by all slaves.

The PEC byte transmitted to each slave is calculated using only

its slave address, command code, and data bytes.

SLAVE 1

ADDRESS

SLAVE 2

ADDRESS

DATA

1 . . . N

COMMAND

CODE

PEC

DATA

1 . . . N

COMMAND

CODE

PEC

SLAVE M

ADDRESS

DATA

1 . . . N

COMMAND

CODE

PEC

= MASTER TO SLAVE

= SLAVE TO MASTER

Figure 28. Group Command Protocol with PEC

CLOCK GENERATION AND STRETCHING

The ADM1266 is always a PMBus slave device in the overall

system; therefore, the device never needs to generate the clock,

which is performed by the master device in the system.

However, the PMBus slave device is capable of clock stretching

to place the master in a wait state. By stretching the SCL signal

during the low period, the slave device communicates to the

master device that it is not ready and that the master device

must wait.

Conditions where the PMBus slave device stretches the SCL line

low include the following:

•

The master device is transmitting at a higher baud rate

than the slave device.

•

The receive FIFO buffer of the slave device is full and must

be read before continuing to prevent a data overflow

condition.

•

The slave device is not ready to send data that the master

has requested.

The slave device can stretch the SCL line only during the low

period. Whereas the I2C specification allows indefinite

stretching of the SCL line, the PMBus specification limits the

maximum time that the SCL line can be held low to 25 ms, after

which the ADM1266 must release the communication lines and

reset its state machine.

START AND STOP CONDITIONS

Start and stop conditions involve serial data transitions while

the serial clock is at a logic high level. The PMBus slave device

monitors the SDA and SCL lines to detect the start and stop

conditions and transition to its internal state machine accordingly.

Figure 29 shows typical start and stop conditions.

Figure 29. Start and Stop Transitions

REPEATED START CONDITION

In general, a repeated start (Sr) condition is the absence of a

stop condition between two transfers. The PMBus communica-

tion protocol makes use of the repeated start condition only when

performing a read access (read byte, read word, and block read).

Other uses of the repeated start condition are not allowed.

GENERAL CALL SUPPORT

The PMBus slave is capable of decoding and acknowledging

a general call address. The PMBus device responds to both its

own address and the general call address (0x00).

All PMBus commands must start with the slave address with the

R/W bit cleared (set to 0), followed by the command code, when

using the general call address to communicate with the PMBus

slave device.

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