User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 14: Serial Configuration Interface Page
403
S e c t i o n 1 4 : S e r i a l C o n f i g u r a t i o n I n t e r f a c e
I
NTRODUCTION
There are two serial configuration interfaces based on the SMBus. The interfaces provide hardware assistance
for simple read and write of slave devices with the part as the bus master. The hardware assistance provides
a subset of the SMBus version 1.1 specification published by the Smart Battery System Implementers Forum
.
The bus clock is programmable, it should be set in the 10-100kHz range for conformance operation, but can
go higher. In particular many devices can use a 400kHz clock.
SMB
US
O
VERVIEW
There are a large number of devices that can be configured using a two wire serial interface with one clock
wire and one data wire. The interface to these devices is basically the same, but there are differences in timing
and electrical specifications. The serial configuration interface is based on version 1.1 of the System
Management Bus (SMBus) standard from the Smart Battery System Implementers Forum (SBS-IF), but has
been generalized to support other devices. This interface only implements a subset, so is not fully compliant
with the SMBus 1.1 specification. This section provides a brief introduction to the interface, for full details see
the SMBus specification that may be found on the SMBus web site at http://www.smbus.org and the related
documents.
Devices on the serial configuration bus may be masters or slaves (or both). The interface is always a master.
Masters can issue commands, provide the clock and control the transfer. Slaves respond to commands from
masters and use the supplied clock. Each slave has a unique 7 bit address (or a few addresses) that the master
uses to select it. Transactions always start with the master sending a byte containing the address of the slave
and a single bit that indicates if the master is reading from the slave or writing to it. Following this one or more
bytes of data are transferred. The SMBus adds a protocol layer above this basic transport.
T
RANSPORT
P
ROTOCOL
Devices connected to the serial configuration bus use open drain outputs on both the clock and data lines. Pull-
up resistors restore the lines high when no device is driving. The inputs should not be pulled above 3.3V.
Both the data (SDA) and clock (SCL) lines are high when idle. In normal operation SDA will only change while
SCL is low. SDA changing with SCL high is used to signal control flags.
shows the three conditions
for bit transfer on the SMBus:
Start
A Start condition is used to indicate the beginning of a transaction. It consists of SDA changing from high (idle)
to low while SCL remains high (idle).
Data bit
A data bit is transferred by putting it on the SDA line with the SCL clock low and pulsing the clock high and
then low with SDA stable. Data bytes are sent with the most significant bit first.