Rainbow Electronics ATtiny43U - Manual (Page 103)

103
8048B–AVR–03/09
<continued>
SPITransfer_loop:
out USICR,r17
in r16, USISR
sbrs r16, USIOIF
rjmp SPITransfer_loop
in r16,USIDR
ret
The code is size optimized using only eight instructions (plus return). The code example
assumes that the DO and USCK pins have been enabled as outputs in DDRA. The value stored
in register r16 prior to the function is called is transferred to the slave device, and when the
transfer is completed the data received from the slave is stored back into the register r16.
The second and third instructions clear the USI Counter Overflow Flag and the USI counter
value. The fourth and fifth instructions set three-wire mode, positive edge clock, count at USITC
strobe, and toggle USCK. The loop is repeated 16 times.
The following code demonstrates how to use the USI as an SPI master with maximum speed
(f
SCK
= f
CK
/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