MP8796B datasheet(28/52 Pages) MPS

28 / 52 page

MP8796B – 16V, 30A, SYNCHRONOUS STEP-DOWN CONVERTER WITH PMBUS

MP8796B Rev. 1.0

www.MonolithicPower.com

1/19/2020

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

•

Read word with PEC

•

Block read with PEC

PMBus Bus Message Format

In Figure 11 on page 29, unshaded cells

indicate that the bus host is actively driving the

bus, and shaded cells indicate that the

MP8796B is driving the bus.

S = Start condition

Sr = Repeated start condition

P = Stop condition

R = Read bit

= Write bit

A = Acknowledge bit (0)

= Acknowledge bit (1)

“A” represents the acknowledge (ACK) bit. The

ACK bit is active low (logic 0) if the transmitted

byte is received successfully by a device.

However, when the receiving device is the bus

master, the acknowledge bit for the last byte

read is logic 1, indicated by A (see Figure 10).

Packet Error Checking (PEC)

The MP8796B PMBus interface supports the

use of a packet error checking (PEC) byte. The

PEC byte is transmitted by the MP8796B during

a read transaction or sent by the bus host to the

MP8796B during a write transaction.

The PEC byte is used by the bus host or the

MP8796B to detect errors during a bus

transaction

(depending

whether

the

transaction is a read or a write). If the host

determines that the PEC byte read during a

read transaction is incorrect, it can decide to

repeat the read if necessary. If the MP8796B

determines that the PEC byte sent during a

write transaction is incorrect, it ignores the

command (does not execute it) and sets a

status flag. Within a group command, the host

can choose to send or not send a PEC byte as

part of the message to the MP8796B.

PMBus Alert Response Address (ARA)

The PMBus alert response address (ARA) is a

special address that can be used by the bus

host to locate any devices that need to talk to it.

A host typically uses a hardware interrupt pin to

monitor the PMBus ALERT pins of a number of

devices. When a host interruption occurs, the

host issues a message on the bus using the

PMBus receive byte, or the receive byte with

PEC protocol.

The special address used by the host is 0x0C.

Any devices that have a PMBus use the ALERT

signal to return their own 7-bit address as the 7

MSBs of the data byte. The LSB value is not

used, and can be either 1 or 0. The host reads

the device address from the received data byte

and proceeds to handle the alert condition.

More than one device may have an active

PMBus

ALERT

signal

and

attempt

communicate with the host. In this case, the

device with the lowest address dominates the

bus and succeeds in transmitting its address to

the host. The device that succeeds disables its

PMBus alert signal. If the host sees that the

PMBus alert signal is still low, it continues to

read addresses until all devices that need to

talk to it have successfully transmitted their

addresses.

Figure 10: Data Transfer over PMBus

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