141
9140CINDCO02/10
ATA8741
23.3.2
SPI Master Operation Example
The following code demonstrates how to use the USI module as a SPI Master:
SPITransfer:
out
USIDR,r16
ldi
r16,(1<
out
USISR,r16
ldi
r16,(1<
SPITransfer_loop:
out
USICR,r16
in
r16, USISR
sbrs
r16, USIOIF
rjmp
SPITransfer_loop
in
r16,USIDR
ret
The code is size optimized using only eight instructions (+ ret). The code example assumes that
the DO and USCK pins are enabled as output in the DDRE Register. The value stored in register
r16 prior to the function is called is transferred to the Slave device, and when the transfer is com-
pleted the data received from the Slave is stored back into the r16 Register.
The second and third instructions clears the USI Counter Overflow Flag and the USI counter
value. The fourth and fifth instruction set Three-wire mode, positive edge Shift Register clock,
count at USITC strobe, and toggle USCK. The loop is repeated 16 times.
The following code demonstrates how to use the USI module as a SPI Master with maximum
speed (f
sck
 = f
ck
/4):
SPITransfer_Fast:
out
USIDR,r16
ldi
r16,(1<
ldi
r17,(1<
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
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