PIC MCU: Triggered Counter from ADC

Question

Basically, I'm looking to program a PIC16F917, so that a particular int will increment by one every time I press a button.

Now I got the ADC up and running, I got loops to count, but I seem to be lacking the knowledge and experience to put it altogether, and latch the input back to zero before the int increases by a number greater than one.

The following code is what I have written so far. The problem with it is that it increments multiple times per push (as many delay cycles that the button was held for), and I'd love to make it only count by one. I was thinking of creating a new function and int but I just can't seem to put it together - much help needed.

Thanks for helping, Ezra

#include <xc.h>
#include "config-bits.h"

#define _XTAL_FREQ 4000000

int result;
int output;

void delay()
{
    int i;

    for(i=0; i<100; i++)
    {
        /*Timer Stuff*/
    }
}

int main()
{
    TRISA=1;                    //Set all pins to input
    TRISB=1;
    TRISC=1;
    TRISD=1;
    TRISE=1;

    TRISDbits.TRISD0 = 0;       //LED output
    ANSELbits.ANS0 = 1;         //Select ADC input

    ADCON0bits.ADFM = 1;        //ADC result is right justified
    ADCON0bits.VCFG = 0;        //Vdd is the +ve reference
    ADCON1bits.ADCS = 0b001;    //Fosc/8 is the conversion clock
                                //This is selected because the conversion
                                //clock period (Tad) must be greater than 1.5us.
                                //With a Fosc of 4MHz, Fosc/8 results in a Tad
                                //of 2us.
    ADCON0bits.CHS =  0;        //select analog input, AN2
    ADCON0bits.ADON = 1;        //Turn on the ADC

    while(1)
    {
        delay();                        //Wait the acquisition time (about 5us).

        ADCON0bits.GO = 1;              //start the conversion
        while(ADCON0bits.GO==1){};      //wait for the conversion to end

        result = (ADRESH<<8)+ADRESL;    //combine the 10 bits of the conversion

        if(result > 512)
        {
            PORTD=output++;
            delay();
        }
        else
        {
            NOP();
        }
    }
}

Answer 1

Use a variable to remember whether the last result was < than 512. Prior to incrementing the output variable, check if the previous result was < 512. Could you have used a digital input rather than an analogue one?

Answer 2

Ok, first a few things. TRISA=1 doesn't put all the PORTA as input as written in the comment, it is equivalent to 0b00000001 so to put all the port as input you have to do TRISA=255 or TRISA=0xFF which equals to 0b11111111.

I don't know how you connected the button to the micro controller but looking to your program it should be something like this:

^{simulate this circuit – Schematic created using CircuitLab}

On this way it should work, you get 5V when the button is pressed and 0V when it is open. However you still can get some inestable voltage at the moment the button is pressed (called bouncing), it looks like this:

So it may seems like the button is pressed multiple times due to this bouncing effect, to solve this you have to take more samples and average them, then use the average result to decide what to do.

But there is another way, which is easier in this case, you just want to read if the switch is pressed ('1') or not ('0') so you don't need and ADC, you can do it just with a digital input, you don't need an analog one. The schematic would be same but the C program would be something like this (let's suppose the switch is connected to RA0):

int main() {
    TRISA=1;                    // Set RA0 as input
    TRISD=0x00;
    PORTD=0x00;

    while(1){
        if(TRISAbits.RA0) {
            PORTD += 1;
            delay();     // We use the delay to avoid the bouncing effect (some milliseconds)
            while(TRISAbits.RA0);
        }
    }
}

Analog inputs are sometimes useful when you have to read a lot of buttons and you want to save some MCU inputs:

In this circuit each button produces a different voltage in the ADC input. So with the analog input you read that voltage and determine which button was pressed.