Timer 0 as External Counter in 8-bit PIC microcontroller

CategoriesElectronics PIC Timers

Timer 0 as External Counter in 8-bit PIC microcontroller

By JuditovaOn 2021-04-282024-01-200 Comments

In the previous post, Timer 0 as an Internal Counter was explained and configured to use the internal clock (i.e. the oscillator) to count a defined quantity of time. However, the External Counter will consider pulses outside the microcontroller, such as push buttons, sensors, transductors, etc. Likewise the Internal Counter, every time as overflow occurs, an interrupt will handle the emergent situation.

Abstract vector created by macrovector – www.freepik.com

Before reading this, you will need to know how to setup MPLAB IDE and how to connect the PICkit to the PIC microcontroller. If not, check them out first. Check out also another projects for beginners in 8-bit PIC microcontrollers.

Requirements for MPLAB to program a PIC Microcontroller

Requirements for Timer 0 as External Counter

Components and Devices

In this tutorial, all the components are Through Hole.

PIC16F628A: 1 unit.
LED, any color or size: 1 unit.
Resistors, 1/2 or 1/4W, 5% tolerance:
- 1KΩ : 1 unit.
- 10KΩ : 1 unit.
Push button, type SPST: 1 unit.

PIC16F628A
Collection of Through Hole (TH) LED. Respecfully borrowed from Wikipedia. https://commons.wikimedia.org/wiki/File:Verschiedene_LEDs.jpg
TH Resistors. Computer vector created by macrovector – www.freepik.com
SPST Push Button

Check out the commercial values of resistors and capacitors here.

PIC microcontroller

For this tutorial, I will use the 8-bit microcontroller PIC16F628A. This one is the very first one that I learned to program. Very practical too. Be sure to select Through Hole or ‘TH' version. Surface Mounted won't fit in the breadboard.

DIP 18 pins Integrated Circuit IC — This is a typical DIP IC with 18 pins

Download the datasheet here. I recommend that you print in reduced A4 all the datasheet because it will serve you as quick reference.

Tools and Machinery

Breadboard: 1 unit.
AC/DC Power Adapter to 5V DC, with at least 500mA: 1 unit.
Jumper or UTP Wires: various.
Multimeter: 1 unit.

Breadboard. Respectfully borrowed from Wikipedia. https://commons.wikimedia.org/wiki/File:400_points_breadboard.jpg
AC/DC adapter
Jumper Wires. Respectfully borrowed from Wikipedia. https://en.wikipedia.org/wiki/File:A_few_Jumper_Wires.jpg
Digital electrical tester on metallic background.

The Design of the Timer 0 as External Counter

Before reading further, make sure that you read how the Timer 0 works and how it is configured. All the details from the step-by-step guide is detailed here.

To make a practical tutorial, let's count 10 times with a push button. Afterwards, an LED will be turned on briefly and it will be turned off back. For this case, Timer 0 register has to be filled near its limit.

Timer 0 is a 8-bit counter. This means it can count from 0 to 255 (i.e. 2^8). But to make it useful, let's set it from 245. It will count with every press of the push button and there will be a overflow (from 255 to ‘256') after the 10 presses. This way, the Timer 0 flag will be set and the task or command will be executed.

Programming the Timer 0 as External Counter

Configuration Bits

On table 6-1, located in page 47, the registers associated with Timer 0 are displayed. Let's start with the Option Register.

Firstly, in the function config() the registers are configured to match the desired operation. To do so, go to page 23 in the datasheet to read about the ‘Register 4-2: Option Register'. The bit T0CS must be set to 1 in order to count from the external source, which is the RA4 pin. Next, the bit PSA must be set to 0 for the prescaler to work with the Timer 0. The Source Edge bit T0SE let us choose between a high-to-low or low-to-high trigger. If your circuit requires a specific option, then select either 1 or 0, respectively. The final bits PS2:PS0 requires to be 0 because the external clock input RA4 and the prescaler requires to have the same output. Therefore, the prescaler 2 will be 000. Now, this is translated into code:

//Timer 0 configuration
T0CS=1;
T0SE=1;
PSA=0;
PS2=0; PS1=0; PS0=0;

The value of the TMR0 has to be preloaded in the configuration and everytime an interrupt occurs.

TMR0=245;

Afterwards, the Timer 0 enable bit must be set, the flag bit reset and finally the Global Interrupt Enable must be set.

TMR0IF=0;
TMR0IE=1;

GIE=1;

Now, it's configured. But what should it do every time that the overflow occurs? Let's deal with the interrupt.

Interrupt Function

If you haven't read about the Interrupt Sources or how to handle Functions in C, check them out first.

Create the function called void __interrupt(void) interruptFunction(). From this place, all the interrupts are handled. Specify the enable bit and flag bit in order to enter the function. In addition, write the Routine_Tmr0() to specify where it should go next if Timer 0 requires the attention.

//Interrupt Sources
  if(TMR0IE && TMR0IF) {
    TMR0IE=0; //Disable Timer 0 interrupt
    Routine_Tmr0();
    TMR0IF=0; //Once attended, erase the interrupt flag from Timer 0
    TMR0IE=1; //Enable Timer 0 interrupt
  }

And now, let's attend the interrupt of Timer 0 caused by the overflow. Let's program a small LED blink, the same way it was done in the previous tutorial.

void Routine_Tmr0(){
 PORTBbits.RB3=1; //LED On
      
 //delay
 unsigned int x;
 x=0;
 while(x<65000) x++;
      
 PORTBbits.RB3=0; //LED Off
 TMR0=245; //Reload the TMR0 value
}

Extra Coding

On MPLab, select ‘Production' tab from the main menu and then clic ‘Set Configuration bits'. I recommend selecting the internal oscillator for simplicity. Select the following options:

 /*-------------------------------------------------------------------
  1. DEFINITION OF BITS, PINS, ALIASES, CONSTANTS
---------------------------------------------------------------------*/
// CONFIG
#pragma config FOSC = INTOSCIO  // Oscillator Selection bits (INTOSC oscillator: I/O function on RA6/OSC2/CLKOUT pin, I/O function on RA7/OSC1/CLKIN)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF      // Power-up Timer Enable bit (PWRT disabled)
#pragma config MCLRE = OFF      // RA5/MCLR/VPP Pin Function Select bit (RA5/MCLR/VPP pin function is digital input, MCLR internally tied to VDD)
#pragma config BOREN = ON       // Brown-out Detect Enable bit (BOD enabled)
#pragma config LVP = ON         // Low-Voltage Programming Enable bit (RB4/PGM pin has PGM function, low-voltage programming enabled)
#pragma config CPD = OFF        // Data EE Memory Code Protection bit (Data memory code protection off)
#pragma config CP = OFF         // Flash Program Memory Code Protection bit (Code protection off)

// #pragma config statements should precede project file includes.
// Use project enums instead of #define for ON and OFF.

#include <xc.h>

In the config() put the pin configuration (1 for everything except for RB3).

//Pin Configuration
TRISA = 0b1111111; //RA7 to RA0.
TRISB = 0b11110111; //RB7 to RB0.
PORTA=0; //Clean PORTA
PORTB=0; //Clean PORTB

Don't forget to add the While(1) in the Main function to make it run forever!

while(1) {

}

Download the code of Timer 0 as External Counter

If you would like to see and read the whole code through, enter your name and e-mail in the form below to download the project. I promise that I won’t send you spam; just relevant content to the blog. If you don’t see any form below, please click here.

Schematic of Timer 0 as External Counter

For reference, here is the schematic used.

Picture of Timer 0 as External Counter

This is how the Timer 0 looks like in the breadboard.

Testing the Timer 0 as External Counter

Finally, a video testifying that it can count until 10. For a short time, a LED is lighten up.

Conclusion

This post has explain how to use Timer 0 to count from external pulses. The code has been written and it's downloadable! The schematic, foto and video has been recorded to show how it is done.

Did you noticed that the counter not always counts exactly 10? It's an inherent problem with push buttons called Switch Bounce or Contact Bounce that can be dealt with hardware or software. More about that in a future tutorial.

Check out two other tutorials about timers: Timer 1 and Multiplexing. The links will be linked here for you when there are written.

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