Intel
®
81341 and 81342—SMBus Interface Unit
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
866
Order Number: 315037-002US
14.3.2.2.2 Stop Phase
A stop condition is generated when SMBus is busy to indicate that its state is changing
to idle. The Stop condition occurs when
SMBDAT
transitions from Low to High while
SMBCLK
remains High.
A stop bit can occur at any point in a data stream. It is not insured to occur after an
ACK from a target (as later waveforms show). The 81341 and 81342 must be able to
accept a stop condition at any time and clean up.
14.3.2.2.3 ACK/NACK
For every 8 bits of data transfer (including address and direction), the receiving agent
must respond with ACK or NACK. An ACK is requires
SMBDAT
= 0 during
SMBCLK
=
1. A NACK requires
SMBDAT
= 1 during
SMBCLK
= 1 as shown below.
During a write cycle, the 81341 and 81342 must drive an ACK after the address/
direction phase, and after the data phase. During a read cycle, the 81341 and 81342
must drive an ACK/NACK after the address/direction phase, and (when ACKed) the
initiator must drive an ACK/NACK after the 81341 and 81342 returns its 8 bits of data.
14.3.2.2.4 Wait States
The receiver (initiator or target) can add wait states, after driving ACK for receiving the
last byte, by driving the
SMBCLK
line low. Further data transfers are delayed until the
receiver stops driving
SMBCLK
low. It is expected the 81341 and 81342 drives the
SMBCLK
line low after receiving data on writes until the write is complete, and after
receiving the direction bit on reads until the read data is ready.
Figure 124. Stop (P) Signaling
SMBCLK
SMBDAT
B6278-01
Figure 125. ACK (A) Signaling
Figure 126. NACK (N) Signaling
SMBCLK
SMBDAT
B6279-01
SMBCLK
SMBDAT
B6280-01