123
8006K–AVR–10/10
ATtiny24/44/84
3.
The master set the first bit to be transferred and releases the SCL line (C). The slave
samples the data and shifts it into the USI Data Register at the positive edge of the SCL
clock.
4.
After eight bits containing slave address and data direction (read or write) have been
transferred, the slave counter overflows and the SCL line is forced low (D). If the slave
is not the one the master has addressed, it releases the SCL line and waits for a new
start condition.
5.
When the slave is addressed, it holds the SDA line low during the acknowledgment
cycle before holding the SCL line low again (i.e., the USI Counter Register must be set
to 14 before releasing SCL at (D)). Depending on the R/W bit the master or slave
enables its output. If the bit is set, a master read operation is in progress (i.e., the slave
drives the SDA line) The slave can hold the SCL line low after the acknowledge (E).
6.
Multiple bytes can now be transmitted, all in same direction, until a stop condition is
given by the master (F), or a new start condition is given.
If the slave is not able to receive more data it does not acknowledge the data byte it has last
received. When the master does a read operation it must terminate the operation by forcing the
acknowledge bit low after the last byte transmitted.
14.3.5
Start Condition Detector
The start condition detector is shown in
Figure 14-6. The SDA line is delayed (in the range of 50
to 300 ns) to ensure valid sampling of the SCL line. The start condition detector is only enabled
in two-wire mode.
Figure 14-6. Start Condition Detector, Logic Diagram
The start condition detector works asynchronously and can therefore wake up the processor
from power-down sleep mode. However, the protocol used might have restrictions on the SCL
hold time. Therefore, when using this feature the oscillator start-up time (set by CKSEL fuses,
of the USISIF bit on
page 125 for further details.
14.3.6
Clock speed considerations
Maximum frequency for SCL and SCK is f
CK / 2. This is also the maximum data transmit and
receive rate in both two- and three-wire mode. In two-wire slave mode the Two-wire Clock Con-
trol Unit will hold the SCL low until the slave is ready to receive more data. This may reduce the
actual data rate in two-wire mode.
SDA
SCL
Write( USISIF)
CLOCK
HOLD
USISIF
DQ
CLR
DQ
CLR