ATtiny261A datasheet(129/292 Pages) ATMEL

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129

8197A–AVR–10/09

ATtiny261A/461A/861A

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.

13.3.5

Start Condition Detector

The start condition detector is shown in Figure 13-6. 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 13-6. 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 in this case the Oscillator start-up time set by the

CKSEL Fuses (see “Clock System” on page 24) must also be taken into the consideration. Refer

to the USISIF bit description on page 131 for further details.

13.3.6

Clock speed considerations

Maximum frequency for SCL and SCK is fCK / 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.

13.4 Alternative USI Usage

The flexible design of the USI allows it to be used for other tasks when serial communication is

not needed. Below are some examples.

SDA

SCL

Write( USISIF)

CLOCK

HOLD

USISIF

CLR

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