6-Timers in PIC

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The microcontroller PIC16F877 has 3 different timers:

• PIC Timer0

• PIC Timer1

• PIC Timer2

PIC16 C Hardware Timers

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PIC Timer0

The Timer0 module timer/counter has the following

features:

• 8-bit timer/counter

• Readable and writable

• 8-bit software programmable prescaler

• Internal (FOSC/4) or external clock select

• Interrupt on overflow from FFh to 00h

• Edge select (rising or falling) for external clock

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BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER

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T0SE: TMR0 Source Edge Select bit

1 = Increment on high-to-low transition on T0CKI pin

0 = Increment on low-to-high transition on T0CKI pin

T0CS: TMR0 Clock Source Select bit

1 = Transition on T0CKI pin

0 = Internal instruction cycle clock (CLKO)

PSA: Prescaler Assignment bit

1 = Prescaler is assigned to the WDT

0 = Prescaler is assigned to the Timer0 module

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Calculating Count, Fout, and TMR0 values

• If using INTERNAL clock, the division is performed as

follow:

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Fout The output frequency after the division.

Tout The Cycle Time after the division.

4 The division of the original clock by 4

Count A numeric value to be placed to obtain the desired output frequency - Fout.

(256 - TMR0) The number of times in the timer will count based on the register TMR0.

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An example of INTERNAL clock

• Suppose we want to create a delay of 0.5 second in the

our program using Timer0. What is the value of Count?

Calculation:

• First, let’s assume that the frequency division by the

Prescaler will be 1:256 and fclk=4MHz. Second, let’s set

TMR0=0. Thus:

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An example of EXTERNAL clock source

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What is the output frequency - Fout, when the external

oscillator is 100kHz and Count=8?

Calculation:

First, let’s assume that the frequency division by the

Prescaler will be 1:256. Second, let’s set TMR0=0. Thus:

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Test circuit

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RTCC (timer0) and interrupts

#include<16f877a.h>

#fuses HS,NOLVP,NOWDT,PUT

#use delay(clock=20000000)

#use rs232(baud=9600,xmit=PIN_C6,rcv=PIN_C7)

#define high start 76

unsigned int8 seconds, high count;

#INT_RTCC //Interrupt procedure

clock_isr() { //called every time RTCC

high_count -= 1; //flips from 255 to 0

if(high_count==0) {

++seconds;

high_count=high_start; //Inc SECONDS counter every

} //76 times to keep time

}

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RTCC (timer0) and interrupts

void main() { //a simple stopwatch program

unsigned int8 start, time;

high_count = high_start;

setup_timer_0( RTCC_INTERNAL | RTCC_DIV_256 );

set_timer0(0);

enable_interrupts(INT_RTCC);

enable_interrupts(GLOBAL);

do {

printf("Press any key to begin.\n\r"); getc(); start = seconds;

printf("Press any key to stop.\r\n"); getc(); time = seconds -

start;

printf("%U seconds.\n\r", time);

} while (TRUE);

} 10

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Timer0-Counter

#include<16f877a.h>



void main() {

unsigned int8 count;;

setup_counters (RTCC_EXT_L_TO_H,RTCC_DIV_1);

set_timer0(0);

do {

if (count !=get_timer0()){

count=get_timer0();

printf("%U times.\n\r", count);

}

} while (TRUE);

}

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TIMER1 MODULE

• The Timer1 module, timer/counter, has the following

features:

• 16-bit timer/counter consisting of two 8-bit registers

(TMR1H and TMR1L)


• Interrupt on overflow from FFFFh to 0000h

Timer1 can operate in one of two modes:

• As a Timer

• As a Counter 14

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DIAGRAM

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TMR1CS: Timer1 Clock Source Select bit

1 = External clock from pin RC0/T1OSO/T1CKI (on the rising edge)

0 = Internal clock (FOSC/4)

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#include<16f877a.h>




#define high_start 10

unsigned int8 seconds, high_count;

#INT_TIMER1 //Interrupt procedure

clock_isr() { //called every time Timer1


if(high_count==0) {

++seconds;



}

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Timer1 and interrupts

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setup_timer_1( T1_INTERNAL | T1_DIV_BY_8 );

set_timer1(3036);

enable_interrupts(INT_TIMER1);


do {


printf("Press any key to stop.\r\n"); getc(); time = seconds - start;


} while (TRUE);

}

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#include<16f877a.h>



void main() {

unsigned int16 count;

set_timer1(3600);

setup_timer_1(t1_external | T1_DIV_BY_1);

delay_ms(500);

count=get_timer1();

do {

if (count !=get_timer1()){

count=get_timer1();

printf("%LU times.\n\r", count);

}

} while (TRUE);

}

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Timer1-Counter

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Timer1-Counter

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Timer2

The Timer2 module, timer/counter, has the following

features:

• Two 8-bit registers (TMR2 and PR2)


• A prescaler and a postscaler

• Connected only to an internal clock

• Interrupt on overflow

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DIAGRAM

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• Timer2 has 2 count registers 8-bits: TMR2 and PR2.

PR2 is a readable and writable register and initialized to

FFh upon Reset.

• Register TMR2 is used to store the "initial" count value

(the value from which it begins to count). Register PR2 is

used to store the "ending" count value (the maximum

value we need/want to reach). ie: using Timer2 we can

determine the started count value, the final count value,

and the count will be between these two values. The

Timer2 increments from 00h until it matches PR2 and

then resets to 00h on the next increment cycle.

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TIMER2 BLOCK

DIAGRAM

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• Prescaler and Postscaler - Timer2 is an 8-bit timer with a

prescaler and a postscaler. Each allows to make

additional division of the frequency clock source.

• Prescaler divides the frequency clock source BEFORE

the counting take place at the register TMR2, thus the

counting inside the TMR2 register is performed based on

the divided frequency clock source by the Prescaler

• The match output of TMR2 goes through a 4-bit

postscaler (which gives a 1:1 to 1:16 scaling inclusive) to

generate a TMR2 interrupt (latched in flag bit, TMR2IF

(PIR1<1>)).

• Postscaler divides the frequency that comes out of the

Comparator again for the last time. 24

TIMER2 BLOCK

DIAGRAM

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Calculate the required

values of the TIMER2:

• Fout – The output frequency after the division.

• Tout – The Cycle Time after the division.

• 4 - The division of the original clock (4 MHz) by 4, when using

internal crystal as clock (and not external oscillator).

• Count - A numeric value to be placed to obtain the desired output

frequency - fout.

• (PR2 – TMR2) - The number of times the counter will count.

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Example and calculation

of how to use TIMER2 • Suppose we want to create a delay of 1 second in the our program

using Timer2. What is the value of Count?

• Calculation:

• First, let's assume that the frequency division by the Prescaler will

be 1:1 and Postscaler will be 1:16. Second, let's set TMR1=0 and

PR2=255. Thus:

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#include<16f877a.h>




#define high_start 125

unsigned int8 seconds, high_count;

#INT_TIMER2 //Interrupt procedure

clock_isr() { //called every time Timer2


if(high_count==0) {

++seconds;



}

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setup_timer_2(T2_DIV_BY_16,0x9c,16); //PR2=0x9C=156, Postscaler=16

set_timer2(0);

enable_interrupts(INT_TIMER2);


do {


printf("Press any key to stop.\r\n"); getc(); time = seconds - start;


} while (TRUE);

}

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6-Timers in PIC