
90
1477K–AVR–08/10
ATtiny26(L)
Start Condition
Detector
The start condition detector is shown in
Figure 49. 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.
When Two-wire mode is selected, the USISIF flag is set (to one) when a start condition is
detected. When output disable mode or Three-wire mode is selected and (USICSx = 0b11 &
USICLK = 0) or (USICS = 0b10 & USICLK = 0), any edge on the SCK pin sets the flag.
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
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.
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.
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.
12-bit Timer/Counter
Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit
counter.
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.
Software Interrupt
The counter overflow interrupt can be used as a software interrupt triggered by a clock strobe.