Using Timer 0 Of PIC16F887 To Make A Retentive Timer Low

Pressing a button could make noise disturbing the MCU digital input. Noise produced by internal contact of the switching button. We can eliminate noise using many methods:

• using an S-R flip flop
• using a passive low pass filter
• using a software delay in the program

With a built-in timer of MCU, we can eliminate the noise and limit the time out of button pressing duration.

In this example, I use timer 0 to counter the duration of a button being pressed. The minimum duration is three seconds. If it's pressed shorter than three seconds, it's ignored.

Any calculation relates to timer 0 interrupt is skipped here, due to the repeating  in the previous post.

Schematic diagram. RD0 and RD1 connect to buttons. RD4 and RD5 connect to
two output LEDs. If any button pressing take longer than three seconds, its
corresponding LED will toggle. Otherwise it will be ignored.

Source code is here:

#include "xc.h"
// PIC16F887 Configuration Bit Settings
// CONFIG1
#pragma config FOSC = XT
#pragma config WDTE = OFF
#pragma config PWRTE = OFF
#pragma config MCLRE = ON
#pragma config CP = OFF
#pragma config CPD = OFF
#pragma config BOREN = ON
#pragma config IESO = ON
#pragma config FCMEN = ON
#pragma config LVP = OFF
// CONFIG2
#pragma config BOR4V = BOR40V
#pragma config WRT = OFF
unsigned char cnt_0=0;
unsigned char cnt_1=0;
void main(){
    PORTD=0x00;
    TRISD=0x0F;
    T0CS=0;
    PSA=0;
    T0IE=1;
    GIE=1;
    T0IF=0;
    TMR0=0;
    while(1){
        /*Check RD0*/
        if(RD0==0){
            cnt_1=0;
            while(RD0==0){
                /*if the press durations
                 is greater than 3 minutes*/
                if(cnt_1>9){
                RD4^=1;
                break;
                }
            }
        }
        /*Check RD1*/
        if(RD1==0){
            cnt_1=0;
            /*if the press durations
             is greater than 3 minutes*/
            while(RD1==0){
                if(cnt_1>9){
                RD5^=1;
                break;
                }
            }
        }
    }
}
void interrupt _ISR(void){
    if(T0IF){
        cnt_0+=1;
        T0IF=0;
    }
    /*325 milli seconds*/
    if(cnt_0>=5){
        cnt_0=0;
        cnt_1+=1;
    }
}

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