Rainbow-electronics ATtiny861_V Manual do Utilizador descarregar pdf (Página 129)

129

2588B–AVR–11/06

ATtiny261/461/861

rjmp SPITransfer_loop

lds 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 DDRA or DDRB Register. 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 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 (fsck = fck/4):

SPITransfer_Fast:

sts USIDR,r16

ldi r16,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)

ldi r17,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)|(1<<USICLK)

sts USICR,r16 ; MSB

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16

sts USICR,r17

sts USICR,r16 ; LSB

sts USICR,r17

lds r16,USIDR

ret

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Rainbow-electronics ATtiny861_V Manual do Utilizador Página 129