за NOP'ы не ручаюсь.
PORTA - for DATA:
PORTB - for SCK
PA7 - MISO
PA0 - MOSI
PB0 - SCK

ldi r17,0 ; 0 sck
ldi r18,0xff ; 1 sck
ldi r16, DATA_OUT
out PORTB,r17 ; prepare SCK
;------- выдача и прием
out PORTA,r16 ; data0 out
out PORTB, r18 ; SCK=1
out PORTB, r17 ; SCK=0
lsr r16
in r16,PORTA
.........................
;-------
out PORTA, r16 ; data7 out
out PORTB, r18 ; SCK=1
out PORTB, r17 ; SCK=0
lsr r16
in r16,PORTA ; data7 in IN_DATA in r16
........