ATTINY87_10 Datasheet, PDF (153/292 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 8K/16K Bytes In-System Programmable Flash and LIN Controller

ATtiny87/ATtiny167
14.3.3
14.3.4
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)
sts USICR,r16
...
SlaveSPITransfer:
sts USIDR,r16
ldi r16,(1<<USIOIF)
sts USISR,r16
SlaveSPITransfer_loop:
lds r16, USISR
sbrs r16, USIOIF
rjmp SlaveSPITransfer_loop
lds 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 is 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 is for initialization only and needs only to be executed
once.These instructions sets Three-wire mode and positive edge USI Data Register clock.
The loop is repeated until the USI Counter Overflow Flag is set.
Two-wire Mode
The USI Two-wire mode is compliant to the Inter IC (TWI) bus protocol, but without slew rate
limiting on outputs and input noise filtering. Pin names used by this mode are SCL and SDA.
8265A–AVR–08/10
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