;--------------------------------------------------------
; Roll Your Own Electronic Lock
; Dhananjay V. Gadre
;--------------------------------------------------------
;ser-dong.asm
;Clock Frequency 3.58 MHz
;--------------------------------------------------------
.include "2323def.inc"

	.def	start_t=r2
	.def	stop-t=r3
	.def	timer-reload=r4
	.def	exor=r5

	.def	temp=rl6
	.def	temp2=rl8
	.def	templ=rl9
	.def	low_del=rl7
	.def	high-del=r22
	.def	count=r2O
	.def	data=r2l

	.def	lfsr-preset=r23
	.def	lfsr_shift=r24
	.def	lfsr_result=r25

	.equ	half-step= -23
	.equ	full-step= -46
	.equ	stop-timer= 0
	.equ	start_timer= 2

	.cseg
	.org	0
		rjmp     RESET	;Reset Handle

RESET:	ldi temp, low(RAMEND)

out SPL, temp             ;Init the Stack Pointer


	ldi	temp, ObOO000110	;configure PORT B for all outputs
	out	DDRB, temp	;except PBO

	ldi	temp, ObOO000001	;PBO is input with pullup
			;PB1 is o/p at 101
	out	PORTB, temp

	ldi	temp, $04
	mov	exor, temp
	ldi	temp, stop-timer	;stop the timer
	out	TCCRO, temp
	out	TCNTO, temp

mov stop-t, temp

ldi temp, start-timer

mov start-t, temp      ;register to start the timer
;with CK/8 as the clock source


;Main Program Loop
;This part receives 2 bytes from the PC as seed and
	;the shift value. calculates	the result and transmits
	;the result to the PC

	main-loop: rcall get-byte	;get seed from the PC
		;data is received in Itempl
	com temp	;account for the RS-232 inversion

	mov lfsr-preset,	temp

	rcall send-byte	;echo the received byte

	rcall get-byte	;get the shift count
	com temp

mov lfsr shift, temp
rcall send-byte	;echo it

rcall calc-result	;using the seed and the shift value ;calculate the result. result is in 
;Ilfsr-result' register

mov data, lfsr-result com data
rcall send-byte ;transmit the result rjmp main-loop




;function: GET-BYTE
;This function gets assembles a byte out of serial ;bits on the PORTB.0 pin, uses the 
TimerO to time ;the duration between bits.  TimerO is programmed ;for 104 us which 
corresponds to bit rate of 9600. ;The byte format is: 1 stop bit, no parity bit and

6
;8 data bits Assembled byte is saved in
;register: 'data'

get-byte:	ldi count, 9
ldi data, 0

wait-foro:	sbic PINB, 0
rjmp wait-foro

wait-forl:	sbis PINB, 0
rjmp wait-forl
ldi temp, half-step out TCNTO, temp
out TCCRO, start-t
still-time:
in temp, TIFR
cpi temp, 2
brne still-time
out TIFR, temp
out TCCRO, stop-t
in temp, PORTB
eor temp, exor
out PORTB, temp
sec
sbis PINB, 0
clc
ror	data
ldi	temp, full-step
out	TCNTO, temp
out	TCCRO, start-t
dec	count
cpi	count, 0
brne	still-time
ldi temp, half-step
out TCNTO, temp
s-time:
in temp, TIFR
cpi temp, 2
brne s-time
out TIFR, temp
out TCCRO, stop-t
mov temp, data
ret

;set carry








7

;function: SEND-BYTE
;This function sends out bits serially on
;bits on the PORTB.1 pin from the register 'data'. ;Function uses the TimerO to time the 
duration ;between bits.  TimerO is programmed for 104 us ;which corresponds to bit rate 
of 9600. ;The byte format is: 1 stop bit, no parity bit and ;8 data bits.

send-byte:	ldi count, 10
in temp, PORTB
eor temp, exor
out PORTB, temp
sbi PORTB, 1
ldi temp, full-step out TCNTO, temp
out TCCRO, start-t
till-time:
in temp, TIFR
cpi temp, 2
brne till-time
out TIFR, temp
out TCCRO, stop-t
in temp, PORTB
eor temp, exor
out PORTB, temp
clc
ror data
brcs its-1
cbi PORTB, 1
rjmp loop-over
its-1:	sbi PORTB, 1
loop-over:	ldi temp, full-step out TCNTO, temp
out TCCRO, start-t dec count
cpi count, 0
brne till time
out TCCRO, stop-t ldi temp, 2
out TIFR, temp
ret


8

;function: LFSR
;A routine to generate a sequence using an 8-bit ;LFSR.  Register Itempl is used as the 8-
bit LFSR ;The Taps for an 8-bit LFSR are at: 1, 2, 3 and 7 for ;maximal length.


lfsr:	mov templ, temp andi templ, OblOO01110 ror templ
mov temp2, templ andi temp2, ObOO000001 ror templ
mov low-del, templ andi low-del, ObOO000001 ror templ
mov high-del, templ andi high-del, ObOO000001 ror templ
ror templ
ror templ
ror templ
eor templ, temp2 eor low-del, high-del eor templ, low-del andi templ, ObOO000001 breq 
shift
sec
rjmp no-shift
shift:    clc
no-shift:
rol temp
ret

;temp has the shifted number



;function:	CALC-RESULT ;cycles the LFSR for a count equal to the content ;of 
Ilfsr-shift' register.  The seed is taken from ;the register Ilfsr-preset'.
;The result is stored in register Ilfsr-result'

calc-result:
cpi lfsr-preset, 0


9

breq its-over
cpi lfsr-shift, 0
breq its-over
get-lfsr:
mov temp, lfsr-preset
rcall lfsr
mov lfsr-preset, temp
dec lfsr-shift
cpi lfsr-shift, 0
brne get-lfsr
its-over:
mov lfsr-result, lfsr-preset
ret