Rainbow Electronics Atmega169L - Manual (Page 180)

180
ATmega169V/L
2514A–AVR–08/02
The following code demonstrates how to use the USI module as a SPI Master with max-
imum speed (fsck = fck/2):
SPITransfer_Fast:
out USIDR,r16
ldi r16,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)
ldi r17,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)|(1<<USICLK)
out USICR,r16 ; MSB
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16
out USICR,r17
out USICR,r16 ; LSB
out USICR,r17
in r16,USIDR
ret
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:
sbis USISR,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 is called is trans-
ferred to the master device, and when the transfer is completed the data received from
the Master is stored back into the r16 Register.