;newswich.asm

;4 LEDs on PORTB, 4 switches on PORTD
;PORTD0 SWITCH -----> PORTB0 LED
;PORTD1 SWITCH -----> PORTB1 LED
;PORTD2 SWITCH -----> PORTB2 LED
;PORTD3 SWITCH -----> PORTB3 LED
;Press a switch and corresponding the LED will lightup
;press another switch and the first LED will go off and 
;the LED corresponding to the new switch will light up
;assembled using Atmel's avrasm assembler.


;the following .inc file should be placed in the same directory as
;this assembly program

.include "1200def.inc"

.cseg
.org 0
         rjmp    RESET           ;Reset Handle
         rjmp    RESET
         rjmp    RESET

RESET:   ldi r16, 0b11111111     ;load register r16 with all 1's
         out DDRB, r16           ;configure PORT B for all outputs

         ldi r16, 0b00000000     ;load register r16 with all 0's
         out DDRD, r16           ;configure PORTD for all inputs

         ldi r16, 255            ;all LEDs off
         out PORTB, r16

loopit:  rcall get_switch        ;call the subroutine to
                                 ;determine which switch is pressed.
                                 ;the subroutine returns the result
                                 ;in register r17
                                 
         out PORTB, r17          ;output the value on PORTB  
         rjmp loopit             ;get more

;------------------------***********-------------------------------
;GET_SWITCH: Subroutine to determine which switch is pressed.
;switch on        return value in r17
;  PD0            0b11111110
;  PD1            0b11111101
;  PD2            0b11111011
;  PD3            0b11110111
;registers destroyed: r18, r19
;subroutines called: delay20ms
;------------------------***********-------------------------------
get_switch:
         in r18, PIND           ;read PIND buffer
         andi r18, $0F          ;
         cpi r18, $0F           ;if no switch is pressed
                                ;then loop back till pressed             
         breq get_switch

         cpi r18, 0b00001110    ;check is SW0 is pressed
         brne not_0             ;if not check more
its_0:   rjmp next_step

not_0:   cpi r18, 0b00001101    ;check is SW1 is pressed
         brne not_1             ;if not check more
its_1:   rjmp next_step

not_1:   cpi r18, 0b00001011    ;check is SW2 is pressed
         brne not_2             ;if not check more
its_2:   rjmp next_step

not_2:    cpi r18, 0b00000111    ;check is SW3 is pressed
         brne get_switch        ;if not some problem, so go back

next_step:
         rcall delay20ms        ;call a debounce delay routine

waitfor_rel:                    ;now wait for the switch to be
         in r19, PIND           ;be released
         andi r19, $0F          ;when the switch is released, all
         cpi r19, $0F           ;PIND0-3 bits will be '1'
         brne waitfor_rel

         rcall delay20ms        ;OK, the switch is released
                                ;debounce it 
         mov r17, r18           ;put the switch code in r17
         ori r17, $F0
         ret                    ;and return
         
;------------------------***********-------------------------------
;DELAY20MS: A 20ms delay subroutine
;Crystal Frequency is 4MHz
;registers destroyed: r21, r20
;------------------------***********-------------------------------
delay20ms:
         ldi r21, 31
outer_loop:
         ldi r20, 255
inner_loop:
         nop
         nop
         nop
         nop
         nop
         nop
         nop
         dec r20
         brne inner_loop
         dec r21
         brne outer_loop
         ret