.include <m168def.inc>

.def temp = r16

.equ SS = PB2
.equ MOSI = PB3
.equ MISO = PB4
.equ SCK = PB5

.cseg
.org 0
jmp RESET
.org SPIaddr
jmp EXT_SPI

EXT_SPI:
reti

RESET:
cli
ldi temp, low(RAMEND)
out SPL, temp
ldi temp, high(RAMEND)
out SPH, temp

ldi temp, 1<<SE
out SMCR, temp

ldi temp, (1<<SCK)|(1<<MOSI)|(1<<SS)|(1<<MISO)
out DDRB, temp
ser temp
out PORTB, temp
ldi temp, (1<<SPIE)|(1<<SPE)|(1<<MSTR)
out SPCR, temp

ldi ZH, high(0x00ff)
ldi ZL, low(0x00ff)
sei

Cycle:

cbi PORTB, SS
nop
nop
nop
nop
nop
nop
out SPDR, ZL
sleep
sbi PORTB, SS
nop
nop
nop
nop
nop
nop
jmp Cycle