|
DETECTING SWITCHES |
One of the first things you have to do in any project is interface the outside to the microcontroller.
This includes things like LEDs, switches, piezos and microphones.
In the CODE PUZZLE project, two switches are placed on a single input. By adding a capacitor and resistors to each switch, we can detect if no-switch, either switch or both switches have been pressed.
This is done in software by discharging the capacitor and looking at the time it takes to charge via one of the resistors.
The circuit diagram is needed to see the component values:
The input line is looked at on a regular basis and since most of the
time is taken up with delays in the switch routine, the chance of
detecting the wrong switch will be small.
The sub-routine "Sw" discharges the capacitor via the 470R and the
switch can be pressed at any during this part of the cycle and have no
effect on the outcome.
If either switch is pressed after about 20% of the start of the next
part of the cycle, no switch will be recognised, so the window for an
incorrect reading does not exist.
If a switch is pressed at the wrong time, it will be picked up during
the next cycle.
Once the 100n has been discharged, the detecting line is changed to an
input and after a delay which is 20% longer than 100n takes to charge, a
reading is made. If the line is HIGH, switch A has been pressed. A
further reading is taken 20% after the time taken for switch B to charge
the capacitor. The micro comes out of the sub-routine with one of three
results.
If "A" is pressed, a LED blinks at 1Hz, If "B" is pressed the LED
blinks at 0.3Hz.
|
;PIC12F629.asm ;CODE PUZZLE - Detects buttons A and B list p=12F629 include "p12f629.inc" __CONFIG _MCLRE_OFF & _CP_OFF & _WDT_OFF & _INTRC_OSC_NOCLKOUT ;Internal osc. ; globals fileA equ 26h fileB equ 27h fileC equ 28h fileD equ 29h fileE equ 2Ah ;bits rp0 equ 5 status equ 03h option_reg equ 81h Start org 0x00 ;program starts at location 000 nop nop nop nop ;NOPs to get past reset vector address nop nop SetUp bsf status,rp0 ;Bank 1 movlw b'10000110' ;Turn off T0CKI, prescaler for TMR0 = 1:128 movwf option_reg bcf status,rp0 ;bank 0 movlw 07h ;Set up W to turn off Comparator ports movwf CMCON ;must be placed in bank 0 clrf GPIO ;Clear GPIO of junk goto Main LED0 bsf status,rp0 ;Bank 1 movlw b'00111111' ;make all input movwf TRISIO ;turn off all LEDs bcf status,rp0 ;bank 0 retlw 00 LED1 bsf status,rp0 ;Bank 1 movlw b'00111001' ;make GP1,2 output movwf TRISIO ;enable outputs bcf status,rp0 ;bank 0 movlw 02h ;make GP1 HIGH, GP2 LOW movwf GPIO ;turn on LED1 retlw 00 ;Delay 0.5 sec Del_1 movlw 02h movwf fileC DelX decfsz fileA,1 ; ,1 denotes the result of the decrement goto DelX decfsz fileB,1 ; is placed in the file goto DelX decfsz fileC,1 goto DelX retlw 00 ;test program to detect switch A and switch B and tell them apart!! Switch bsf status,rp0 ;Bank 1 bcf TRISIO,0 ;make GP0 output bcf status,rp0 ;bank 0 bcf GPIO,0 ;make switch-line LOW Sw1 decfsz fileA,1 ; short delay while 100n discharges goto Sw1 bsf status,rp0 ;Bank 1 bsf TRISIO,0 ;make GP0 input bcf status,rp0 ;bank 0 movlw 02 movwf fileB Sw2 movlw 0A0h movwf fileA Sw3 decfsz fileA,1 ; delay while 100n charges via switch goto Sw3 decfsz fileB,1 goto Sw2 btfsc GPIO,0 ;is switch-line HIGH? retlw 01 ;yes - switch A pressed Sw4 decfsz fileA,1 ; delay while 100n charges via switch goto Sw4 btfsc GPIO,0 ;is switch-line HIGH? retlw 02 ;yes - switch B pressed retlw 00 ;no Main call Switch ;see if button A is pressed movwf fileD ;put result of switch into file D btfss fileD,0 ;see if Sw A is pressed goto Main2 call LED1 ;turn on LED 1 call Del_1 call LED0 ;turn off LED 1 call Del_1 goto Main Main2 btfss fileD,1 ;see if Sw B is pressed goto Main call LED1 ;turn on LED 1 call Del_1 call Del_1 call Del_1 call LED0 ;turn off LED 1 call Del_1 call Del_1 call Del_1 goto Main ;Results: B is just at the point of detection ; with 2 x fileB and fileA = 0FFh. ; Use 1 x fileB and fileA = 0FFh ;Button A is not detected with fileA less than D0h ;and 1 x fileB ;Use 2 x fileB and fileA = 0Ah end |
28/8/07