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Data Sheet

ADM1266

Rev. A | Page 29 of 62

PMBus DIGITAL COMMUNICATION

The PMBus slave with packet error checking (PEC) allows a

device to interface to a PMBus compliant master device, as

specified by the PMBus Power System Management Protocol

Specification (Revision 1.2, September 6, 2010). The PMBus slave is

a 2-wire interface that can be used to communicate with other

PMBus compliant devices and is compatible in a multimaster,

multislave bus configuration. The PMBus slave can communicate

with master PMBus devices that support packet error checking

(PEC), as well as with master devices that do not support PEC.

The pull-up resistors on the PMBus pins must not be connected

to AVDD_CAP. If another device on the PMBus line provides a

strong pull-down AVDD_CAP, the ADM1266 shuts down or

enters UVLO.

PMBus FEATURES

The function of the PMBus slave is to decode the command

sent from the master device and to respond as requested.

Communication is established using an I2C like 2-wire interface

with a clock line (SCL) and data line (SDA). The PMBus slave is

designed to externally move blocks of 8-bit data (bytes) while

maintaining compliance with the PMBus protocol. The PMBus

protocol is based on the SMBus Specification (Version 2.0,

August 2000). The SMBus specification is, in turn, based on the

Philips I2C Bus Specification (Version 2.1, January 2000). The

PMBus incorporates the following features:

•

Slave operation on multiple device systems

•

7-bit addressing

•

100 kbps and 400 kbps data rates

•

PEC

•

Support for the group command protocol

•

Support for the alert response address protocol with

arbitration

•

General call address support

•

Support for clock low extension (clock stretching)

•

Separate multiple byte receive and transmit first in, first

out (FIFO)

•

Extensive fault monitoring

OVERVIEW

The PMBus slave module is a 2-wire interface that can be used

to communicate with other PMBus compliant devices. Its transfer

protocol is based on the Philips I2C transfer mechanism. The

ADM1266 is always configured as a slave device in the overall

system. The ADM1266 communicates with the master device

using one data pin (SDA) and one clock pin (SCL). Because the

ADM1266 is a slave device, it cannot generate the clock signal.

However, the ADM1266 is capable of clock stretching the SCL

line to put the master device in a wait state when the ADM1266

is not ready to respond to the request of the master.

Communication is initiated when the master device sends a

command to the PMBus slave device. Commands can be read

or write commands. Data is transferred between the devices in a

byte wide format. Commands can also be send commands. The

command is executed by the slave device upon receiving the

stop bit. The stop bit is the last bit in a complete data transfer, as

defined in the PMBus/SMBus/I2C communication protocol.

During communication, the master and slave devices send

acknowledge or no acknowledge bits as a method of

handshaking between devices.

In addition, the PMBus slave on the ADM1266 supports PEC to

improve reliability and communication robustness. The ADM1266

can communicate with master PMBus devices that support PEC,

as well as with master devices that do not support PEC. See the

SMBus Specification (Version 2.0) for a more detailed description of

the communication protocol.

When communicating with the master device, it is possible for

illegal or corrupted data to be received by the PMBus slave

device. In this case, the PMBus slave device responds to the

invalid command or data, as defined by the PMBus specification,

and indicates to the master device that an error or fault condition

has occurred. This method of handshaking can be used as a first

level of defense against inadvertent programming of the slave

device that can potentially damage the chip or system.

The PMBus specification defines a set of generic PMBus

commands that are recommended for a power management

system. However, each PMBus device manufacturer can choose

to implement and support certain commands as the manufacturer

deems fit for a specific system. In addition, the PMBus device

manufacturer can choose to implement manufacturer specific

commands with functions not included in the generic PMBus

command set.

TRANSFER PROTOCOL

The PMBus slave follows the transfer protocol of the SMBus

Specification (Version 2.0), which is based on the fundamental

transfer protocol format of the Philips I2C Bus Specification

(Version 2.1). Data transfers are byte wide, lower byte first. Each

byte is transmitted serially, most significant bit (MSB) first.

Figure 19 shows a basic transfer.

7-BIT

ADDRESS

R/W

...

8-BIT DATA

= MASTER TO SLAVE

= SLAVE TO MASTER

Figure 19. Basic Data Transfer

For an in depth description of the transfer protocols, see the

SMBus and I2C specifications.

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