i2cwait:
	in r17, TWCR
	sbrs r17, TWINT
	rjmp i2cwait
; r16 : addr
i2cstart:
	ldi r17, (1 << TWSTA) | (1 << TWEN) | (1 << TWINT)
	out TWCR, r17
	rcall i2cwait
	out TWDR, r16
	andi r17, ~(1 << TWSTA)
	out TWCR, r17
	rcall i2cwait
; r16: data 
writebyte: