Timer0 - PIC18F2420/2520/4420/4520

PIC 18f2520 microcontroller timer0

In my previous chapter, we made a LED blinking program with an undesired time delay during on-off. Now we are going to control LED with accurate time delay. For this we would know that how timer works?

The Timer0 module incorporates the following features:

• Software selectable operation as a timer or counter in both 8-bit or 16-bit modes

• Readable and writable registers

• Dedicated 8-bit, software programmable prescaler

• Selectable clock source (internal or external)

• Edge select for external clock

• Interrupt-on-overflow

Timer0 has a specific 8bits register T0CON to control the timer functionality. See controller datasheet for detailed description.

Bit7 – switch the timer on or off.

·       If  bit is set to 0 timer will be off condition, if bit set as 1 timer will in on condition. Either we can set all of bits in a time and/or set separately one.

T0CONbits.TMR0ON = 1;      // Start the timer

Or

T0CON = 0x80;   // Starts the timer but changes to all other bits to 0

So the above codes will be suitable in all of need.

Bit6 – Selection of timer/counter bits mode.

·       Timer0 can count in 8 bit or 16 bit mode. It depends on your need that how much time delay you need. With 16 bits of mood, we can set a long time delay and other facility is also available that is prescaler assignment.

T0CONbits.T08BIT = 1;          // 16 bits mode.

T0CONbits.T08BIT = 0;          // 8 bits mode.

Bit5 – Timer clock selection bit

·       Timer0 can be operate with internal or external clock source. Controller pin number 6 (T0CKI) is external clock source pin for timer increment. Every rising or falling edge increase time as per Bit4 (T0SE) selection.

T0CONbits.T0CS = 1;   //External clock select.

T0CONbits.T0CS = 0;   //Internal clock select.

Bit4 – Timer clock source edge selection bit

T0CONbits.T0SE = 1;    // Increment on high-to-low transition on T0CKI pin.

T0CONbits.T0SE = 0;    // Increment on low-to-high transition on T0CKI pin.

Bit3 – Prescaling selection

·       We can assign prescaling for our suitable need. We need set this to 1 then Bit0-2 prescaler assignment will work otherwise prescaler will not work.

T0CONbits.PSA = 1;     //Prescaler selection on.

T0CONbits.PSA = 0;     //Prescaler selection off.

Bit0-2 are prescaler value selection bits.

We can define timer starting value by TMR0L and TMR0H registers. If we are using 8 bit mode, we would use TMR0L register. If we are using 16 bit mode, both registers will use. The TMR0 interrupt is generated when the TMR0 register overflows from FFh to 00h in 8-bit mode, or from FFFFh to 0000h in 16-bit mode.

The circuit diagram will be same as my previous LED bilking circuit. Even after that i am enclosing circuit.

Circuit diagram

We are going to use 20Mhz external oscillator for system clock and we would define TMR0 register’s bits base on 20Mhz frequency. You can set register value bit by bit or can define in a stoke.

Now comes most important part of timer is Calculating delay time. There are the two method to calculate delay time. Fist is calculate yourself by given blow formulas or by any software like PIC TIMER CALCULATOR.

Exe :- Time delay 1 second with 16bit mode

          Clock Period (T) = 1/Frq = 1/20000000 = 0.00000005 s = 0.05μs

          Instruction cycle clock (Ic) period  = 0.05x4 = 0.2μs

          For one second time delay required instruction cycles

 = 1/0.0000002 = 5000000  Ic

With Prescaler  1:128 = 5000000/128 = 39063

Counting start value =  65536 – 39063 = 26473

26473 in Hexadecimal = 6769

Initial loading value of TMR0H  =  67

                                      TMR0L  =   69

I trusty say that your timer fundamental will be clear and now you will be able to make your timer codes.

Codes

#include <p18f2520.h>

#define LED PORTCbits.RC0

void delay_1_sec(void);   //One second delay time function.

void main()

{

            TRISC = 0;     // PORTC as output.

            LED = 0;         // Turn Off led at staring.

           

            //Configure timer bits.

            T0CONbits.T08BIT = 1;     // 16 bits mode.

            T0CONbits.T0CS = 0;         //Internal clock select.

            T0CONbits.PSA = 1;           //Prescaler selection on.

           

            //Prescaler 1:128 selection bits.

            T0CONbits.T0PS2 = 1;

            T0CONbits.T0PS1 = 1;

            T0CONbits.T0PS0 = 0;

           

            while(1)

            {

                        delay_1_sec();        //Call delay time function.

                        LED = 1;

                        delay_1_sec();        //Call delay time function.

                        LED = 0;

            }          

}

void delay_1_sec()

{

            TMR0H = 0x67;       // Timer higher register value.

            TMR0L = 0x69;        // Timer higher register value.

            T0CONbits.TMR0ON = 1; // Turn on timer.

            while(INTCONbits.TMR0IF == 0);          // Wait until interrupt generate.

            T0CONbits.TMR0ON = 0; // Turn off timer.   

            INTCONbits.TMR0IF = 0;  // Clear interrupt flag.

}

Download Project file :- https://drive.google.com/file/d/0B886Kbl42IVuYTF0T2tzWl9MNDA/view?usp=sharing