Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
865
SMBus Interface Unit—Intel
®
81341 and 81342
14.3.2
SMBus Signaling
14.3.2.1 Overview
The SMBus interface includes a pair of signals:
SMBCLK
(clock) and
SMBDAT
(serial
data).
SMBCLK
provides the timing mechanism for data transfers. The SMBus master
always drives
SMBCLK
. The 81341 and 81342 may optionally extend
SMBCLK
low
time by driving it low to meet setup timings on the SMBus.
An initiator starts a transfer over the SMBus when it is free. Details of how initiators
arbitrate are not described here. The current initiator communicates to the desired
target through a unique 7-bit address to the target, sent MSb to LSb. All devices
monitor the generated address after detecting the start condition. Once seven address
bits are received, all targets compare the received address with their own and the
target slave finds a match.
The next data bit from the initiator indicates the transfer direction. A value of ‘1’
indicates that the target needs to transfer data to the initiator (read). Data transfers
over SMBus are performed in 8-bit chunks. Data is transferred from MSb to LSb.
14.3.2.2 Waveforms
The timing relationship between
SMBDAT
and
SMBCLK
is defined such as the
SMBDAT
value must be valid through the duration of
SMBCLK
being in High state. The
following diagram illustrates data transfer:
14.3.2.2.1 Start Phase
A start condition is generated when SMBus is idle to indicate that its state is changing
to busy. The start condition occurs when
SMBDAT
transitions from High to Low while
SMBCLK
remains High. The SMBus protocol also allows a master to “Repeat Start”,
meaning that a new transfer is started by the same master, without a stop condition.
Figure 122. Basic SMBus Transfer Waveform
Valid
Valid
Valid
SMBCLK
SMBDAT
B6276-01
Figure 123. Start (S) / Repeat Start (Sr) Signaling
SMBCLK
SMBDAT
B6277-01