;Dongle1.asm

;Dhananjay V. Gadre
;27th July 1999
;1st August 1999

;Synopsis: 
;---------

;Security Dongle
;Implements an electronic lock based on LFSR. For illustration
;8-bit LFSR has been implemented. For real applications, longer
;length shift registers could be used.

;The controller is connected to the parallel port for power
;as well as for communication. 

;I/O signals: PB0 ---> Data in/out
;             PB1 ---> strobe_in
;             PB2 ---> ACK_out

;At reset, PB0 is put in input mode to receive 2 bytes. First
;byte is a shift register preset value and the second byte
;is shift count. The program loads the shift register with the
;first byte and then rotates the shift register for a count
;equal to the second byte. AFter that PB0 is put in output
;mode and the result is shifted out.

;Data is transferred using a handshake protocol.


;Any 8 pin (or more) processor can be used. 


.include "2323def.inc"

.equ DATAIO=0
.equ ACK=2
.equ STROBE=1

.def save_status = r0 ;used to store the SREG during an ISR

.def sundry=r16

.def count=r17        

.def low_del=r18      ;delay variable

.def high_del=r19     ;another delay variable

.def temp=r20         ;temperory variable

.def temp1=r21        ;another temperory variable

.def temp2=r22        ;yet another temporary variable

.def lfsr_preset=r23
.def lfsr_shift=r24
.def lfsr_result=r25

.cseg

.org 0
         rjmp    RESET           ;Reset Handle


RESET:   ldi sundry, low(RAMEND)
         out SPL, sundry        ;Init the Stack Pointer 
 

         ldi sundry, 0b00011100 ;PB0, PB1=i/p; rest o/p
         out DDRB, sundry       ;configure PORTB

         sbi PORTB, ACK        ;indicate not ready

main_loop: rcall get_byte
           mov lfsr_preset, temp

           rcall get_byte
           mov lfsr_shift, temp

           rcall calc_result
           rcall send_result
           rjmp main_loop 
           
get_byte:  ldi temp, 0
           mov temp1, temp
           mov temp2, temp
           cbi DDRB, DATAIO      ;change mode of PB0 to input
           sbi PORTB, DATAIO
get_bit:   cbi PORTB, ACK        ;now ready to take data
stb_0:     sbis PINB, STROBE
           rjmp stb_0
           sbi PORTB, ACK
           in temp, PINB
           andi temp, 1
           lsl temp1
           or temp1, temp
stb_1:     sbic PINB, STROBE
           rjmp stb_1
           inc temp2
           cpi temp2, 8
           brne get_bit
           mov temp, temp1
           ret

           
send_result:
           ldi temp, 0
           mov temp1, lfsr_result
           mov temp2, temp
           sbi DDRB, DATAIO
send_bit:  cbi PORTB, ACK
nstb_0:    sbis PINB, STROBE
           rjmp nstb_0
           rol temp1
           brcs its_1
           cbi PORTB, DATAIO
           rjmp skip_over
its_1:     sbi PORTB, DATAIO
skip_over: sbi PORTB, ACK
           sbic PINB, STROBE
           rjmp skip_over
           inc temp2
           cpi temp2, 8
           brne send_bit
           ret  



;A routine to generate random number sequence using LFSR
;This is an 8-bit LFSR
;temp register 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 temp1, temp
         andi temp1, 0b10001110
         ror temp1
         mov temp2, temp1
         andi temp2, 0b00000001
         ror temp1
         mov low_del, temp1
         andi low_del, 0b00000001
         ror temp1
         mov high_del, temp1
         andi high_del, 0b00000001
         ror temp1
         ror temp1
         ror temp1
         ror temp1
         eor temp1, temp2
         eor low_del, high_del
         eor temp1, low_del
         andi temp1, 0b00000001
         breq shift
         sec
         rjmp no_shift
shift:   clc
no_shift:      
         rol temp        ;temp has the shifted number
         ret

calc_result:
         cpi lfsr_preset, 0
         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