Datasheet

129
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
16.3.5 Start Condition Detector
The start condition detector is shown in Figure 16-6 on page 128. The SDA line is delayed (in
the range of 50 to 300ns) 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 pro-
cessor 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 25) must
also be taken into consideration. See the USISIF bit description in “USISR – USI Status Reg-
ister” on page 130 for further details.
16.3.6 Clock speed considerations
Maximum frequency for SCL and SCK is f
CK
/4. 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 SCL low until the slave is ready to receive more data. This may reduce the
actual data rate in two-wire mode.
16.4 Alternative USI Usage
When the USI unit is not used for serial communication, it can be set up to do alternative tasks
due to its flexible design.
16.4.1 Half-duplex Asynchronous Data Transfer
By utilizing the Shift Register in Three-wire mode, it is possible to implement a more compact
and higher performance UART than by software only.
16.4.2 4-bit Counter
The 4-bit counter can be used as a stand-alone counter with overflow interrupt. Note that if the
counter is clocked externally, both clock edges will generate an increment.
16.4.3 12-bit Timer/Counter
Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit
counter.
16.4.4 Edge-Triggered External Interrupt
By setting the counter to maximum value (F), it can function as an additional external interrupt.
The overflow flag and interrupt enable bit are then used for the external interrupt. This feature
is selected by the USICS1 bit.
16.4.5 Software Interrupt
The counter overflow interrupt can be used as a software interrupt triggered by a clock strobe.