102
7598H–AVR–07/09
ATtiny25/45/85
Referring to the timing diagram (Figure 16-5.), a bus transfer involves the following steps:
1.
The a start condition is generated by the Master by forcing the SDA low line while the
SCL line is high (A). SDA can be forced low either by writing a zero to bit 7 of the Shift
Register, or by setting the corresponding bit in the PORT Register to zero. Note that the
Data Direction Register bit must be set to one for the output to be enabled. The slave
device’s start detector logic (Figure 16-6.) detects the start condition and sets the
USISIF Flag. The flag can generate an interrupt if necessary.
2.
In addition, the start detector will hold the SCL line low after the Master has forced an
negative edge on this line (B). This allows the Slave to wake up from sleep or complete
its other tasks before setting up the Shift Register to receive the address. This is done
by clearing the start condition flag and reset the counter.
3.
The Master set the first bit to be transferred and releases the SCL line (C). The Slave
samples the data and shift it into the Serial Register at the positive edge of the SCL
clock.
4.
After eight bits are transferred containing slave address and data direction (read or
write), 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.
If the Slave is addressed it holds the SDA line low during the acknowledgment cycle
before holding the SCL line low again (i.e., the Counter Register must be set to 14
before releasing SCL at (D)). Depending of 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 force the
acknowledge bit low after the last byte transmitted.
Figure 16-6. Start Condition Detector, Logic Diagram
16.2.5
Start Condition Detector
The start condition detector is shown in Figure 16-6. 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.
The start condition detector is working asynchronously and can therefore wake up the processor
from the Power-down sleep mode. However, the protocol used might have restrictions on the
SCL hold time. Therefore, when using this feature in this case the Oscillator start-up time set by
the CKSEL Fuses (see
“Clock Systems and their Distribution” on page 21
) must also be taken
into the consideration. Refer to the USISIF bit description on page 104 for further details.
SDA
SCL
Write( USISIF)
CLOCK
HOLD
USISIF
D Q
CLR
D Q
CLR
