Datasheet

126
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
out USICR,r17
in r16,USIDR
ret
16.3.3 SPI Slave Operation Example
The following code demonstrates how to use the USI module as a SPI Slave:
init:
ldi r16,(1<<USIWM0)|(1<<USICS1)
out USICR,r16
...
SlaveSPITransfer:
out USIDR,r16
ldi r16,(1<<USIOIF)
out USISR,r16
SlaveSPITransfer_loop:
in r16, USISR
sbrs r16, USIOIF
rjmp SlaveSPITransfer_loop
in r16,USIDR
ret
The code is size optimized using only eight instructions (+ ret). The code example assumes
that the DO is configured as output and USCK pin is configured as input in the DDR register.
The value stored in register r16 prior to the function being called is transferred to the master
device, and when the transfer is completed, the data received from the master is stored back
into the r16 register.
Note that the first two instructions are for initialization only and need only to be executed once.
These instructions set the three-wire mode and positive edge shift register clock. The loop is
repeated until the USI counter overflow flag is set.