Now that we learned how to get output form the PIC ( flashing LED ) , we 'll learn to input the
Microcontroller by a push button . This will give you an idea to get order from the user to do an
action ( open door, turn-off lights, ......... ) .
; Button.ASM;******************************************************
list p=16f84
include "p16f84.inc"org 0x00goto start
org 0x20
start
bcf INTCON,7
movlw 0x00
bsf STATUS,5
movwf TRISA
bcf STATUS,5
movlw 0xFF
bsf STATUS,5
movwf TRISB
bcf STATUS,5
againbtfss PORTB,0
call LED_ON
call LED_OFF
goto again
LED_ON
movlw 0xFF
movwf PORTA
goto again
LED_OFF
movlw 0x00
movwf PORTA
goto again
end;***********************************************
The program makes the LED is ON when the Button is pressed , LED off when Button is released.
The only new command here is BTFSS which checks if the button is pressed or released.
What it makes is Bit Test F Skip if Set where F is a bit in register. This command is a
bit-oriented command. It means it deals with one bit of the register.
Build the following circuit in Proteus 7 ISIS :
Follow the steps you learned before to add the source code to the circuit and configure it , then
start simulation. Of course in this time you 'll add the Button.ASM source code.
Now , we 'll write the same program in C
//******************************************************
#include"pic.h"
main()
{
unsigned char i;
TRISA = 0 ; // Make PORTB output
TRISB = 0xFF ; // Make PORTB input
PORTA = 0 ; // Initialize PORTB
for(;;) // This is the infinite loop that keeps the PIC running
{
if ( PORTB == 0 )
PORTA = 1;
else
PORTA = 0 ;
}
}
//************************************************************
Note that the logic is inverted because the button by default inputs one to the PIC and when pressed inputs zero.
Again , you can configure the project for the C source code from this lesson.
You can download the project files and source code from here.
Enjoy.
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