ADP1055ACPZ-R7 datasheet(49/140 Pages) AD

49 / 140 page

Data Sheet

ADP1055

Rev. A | Page 49 of 140

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, command} separated by a repeated start (Sr) bit (see

Figure 78). 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.

Note that the PEC byte transmitted to each slave is calculated

using only its slave address, command code, and data bytes.

Figure 78. Group Command Protocol with PEC

CLOCK GENERATION AND STRETCHING

The ADP1055 is always a PMBus slave device in the overall

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

which is done by the master device in the system. However, the

PMBus slave device is capable of clock stretching to put 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:



Master device is transmitting at a higher baud rate than the

slave device.



Receive FIFO buffer of the slave device is full and must be

read before continuing to prevent a data overflow

condition.



Slave device is not ready to send data that the master has

requested.

Note that the slave device can stretch the SCL line only during the

low period. Also, whereas the I2C specification allows indefinite

stretching of the SCL line, the PMBus specification limits the

maximum time that the SCL line can be stretched, or held low,

to 25 ms, after which the ADP1055 must release the communica-

tion 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 its internal state machine accordingly.

Figure 79 shows typical start and stop conditions.

Figure 79. 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 communication

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).

Note that all PMBus commands must start with the slave address

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

This is also true when using the general call address to communi-

cate with the PMBus slave device. The only exception to this

rule is when the SMBALRT alert response address is used.

ALERT RESPONSE ADDRESS (ARA)

If a PMBus slave device supports the SMBALRT hardware pin

to interrupt the master on a fault condition, the SMBus Alert

Response Address Protocol must be supported to allow commu-

nication between the master and slave on the device that triggers

the fault.

When the SMBALRT pin on the slave is asserted, the master

queries the address of the slave device that triggered the fault by

sending the alert response address (0001 to 100x). In response

to this address, the slave with the asserted SMBALRT pin

acknowledges (ACKs) the address and responds with its own

slave address (7-bit address and plus 0). If multiple slave devices

have their SMBALRT pins asserted, the slave with the lowest

address wins the arbitration and subsequently deasserts its

SMBALRT pin.

Figure 80. ARA Protocol with PEC

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

7-BIT

ARA

SLAVE

ADDRESS

PEC

BYTE

= MASTER-TO-SLAVE

= SLAVE-TO-MASTER

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