BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
404
Section 14: Serial Configuration Interface
Document
1250_1125-UM100CB-R
Stop
A stop condition signals the end of a transaction. It consists of SDA making a transition from low to high (idle)
while SCL is high (idle).
Figure 83: SMBus Signaling Start, Data Transfer and Stop
A master will initiate a transfer by sending a start condition. This causes all slaves to listen to the next byte
which includes the 7 bit address and a single read/write bit. Following this the master sets SDA idle and pulses
SCL once more. If a slave matches the address it will acknowledge by holding SDA low. Bytes are then
transferred in the direction marked by the read/write bit, with the receiver (slave during a write, master during
a read) acknowledging after each. Bytes continue to be transferred until the slave does not acknowledge a byte
or the master decides all bytes have been transferred. The transaction is then ended by the master signalling
a stop, which returns both lines to idle.
There may be multiple masters on the bus. Most of the time they will see the start and stop framing of other
masters to avoid collision, however if two masters start at the same time they may collide. In this case the first
time their SDA signals differ the master that drives the line to zero wins arbitration and the other must back off.
When forced to back off the interface will retry after it sees a stop condition, or if SCL and SDA are both high
for 50 us (this is defined as an idle condition).
The timing specification for slave devices tends to vary with frequency. To comply with both the standard
timings at 100 kHz and typical timings at 400 kHz the interface produces a SCL clock signal with a 9:7 ratio of
time low:time high.
The slave device is permitted to slow the clock by stretching the low period. If a slave has stretched the clock
for 25 ms then an error has occurred and the master will abort the transfer.
SDA
SCL
D7
D6
D5
D4
D3
D2
D1
D0
A/N
Start
Stop
Ack/Nack
from
Receiver
Byte of data transferred
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