3 chapter 3 timer counter modules

-Timer: use to count the time -Counter: use to count the event or some other things -The capacity of Timer/Counter is based on the number of bits they have.. -Timer: use to count the tim

CHAPTER 3

TIMERS – COUNTERS MODULES

Dr Vo Tuong Quan

TIMER - COUNTER

Definition

-The operations of Timer and Counter in microcontroller are similar

-Timer: use to count the time

-Counter: use to count the event or some other things

-The capacity of Timer/Counter is based on the number of bits they have

Ex: Timer 8 bits  can be count from 0 to 255

Timer 16 bits  can be count from 0 to 65535

Timer 32 bits  can be count from 0 to 4294967296

-Timer/Counter have two modes: Count Up/ Count Down

- Timer needs a clock source, for example of the clock source

of 10KHz is input to a timer then one increment will take 100uS (micro second) This clock source can be obtained from the MCU clock

TIMER - COUNTER

Definition

-The operations of Timer and Counter in microcontroller are similar

-Timer: use to count the time

-Counter: use to count the event or some other things

-The capacity of Timer/Counter is based on the number of bits they have

Ex: Timer 8 bits  can be count from 0 to 255

Timer 16 bits  can be count from 0 to 65535

Timer 32 bits  can be count from 0 to 4294967296

-Timer/Counter have two modes: Count Up/ Count Down

- Timer needs a clock source, for example of the clock source

of 10KHz is input to a timer then one increment will take 100uS (micro second) This clock source can be obtained from the MCU clock

TIMER - COUNTER

Prescaler

The function of prescaler is to divide the CPU clock to obtain a smaller frequency Some typicle prescaler division factors:

256 128 64 32 16 8 4 2 1 (Prescaler by-passed)

What is watchdog timer?

A timer that monitors how long it takes the MCU to complete a scan Watchdog timers output an error message if the MCU

scan takes too long.

TIMER - COUNTER

Example: Using Timer of pic18f4520

TIMER0 control rsegister

1 1

1 1

1 1

1 1

Initial

Value

PS0 PS1

PS2 PSA

T0SE T0CS

T08BIT TMR0ON

Name

T0CON

BIT7 - TMR0ON: Timer0 On, set this to 1 to start the timer.

BIT6 - T08BIT: =1 for 8 bit mode and =0 for 16 bit mode.

BIT5 - T0CS: Timer0 clock source =1 for T0CLK pin input i.e

counter mode Set to 0 for internal instruction clock

BIT4 - T0SE: Used in counter mode only Please see

datasheet for details

BIT3 - PSA: Prescaler Assignment Set this to 0 to assign

prescaler or 1 to by pass it

BIT2 to BIT0 - PS2 to PS0: Prescaler Division factor select

TIMER - COUNTER

Divide by 256

1 1

1

Divide by 128

0 1

1

Divide by 64

1 0

1

Divide by 32

0 0

1

Divide by 16

1 1

0

Divide by 8

0 1

0

Divide by 4

1 0

0

Divide by 2

0 0

0

PS0 PS1

PS2

TIMER - COUNTER

// Using FOSC = 20MHZ, Timer 8 bit

unsigned char counter=0; //Overflow counter

void main()

{

//Setup Timer0

T0PS0=1; //Prescaler is divide by 256

T0PS1=1;

T0PS2=1;

PSA=0; //Timer Clock Source is from Prescaler

T0CS=0; //Prescaler gets clock from FMCU (5MHz)

T08BIT=1; //8 BIT MODE

TMR0IE=1; //Enable TIMER0 Interrupt

GIE=1; //Enable INTs globally

TMR0ON=1; //Now start the timer

//Set RB1 as output because we have LED on it

TRISB&=0B11111101;

while(1); //Infinited loop

}

Example code:

TIMER - COUNTER

//Main Interrupt Service Routine (ISR)

void interrupt ISR()

{

//Check if it is TMR0 Overflow ISR

if(TMR0IF) //Interrupt Flag

{

//TMR0 Overflow ISR

counter++; //Increment Over Flow Counter

if(counter== 76) //How to get the 76 value? How many second to delay?

{

if(RB1==0)

RB1=1;

else

RB1=0;

counter=0; //Reset Counter

}

//Clear Flag

TMR0IF=0;

TMR0ON=1; } }

TIMER - COUNTER

How to get the value 76 of counting value?

Prescaler = FMCU/256 (Note: FMCU= Fosc/4)

As our FMCU=20MHz/4 (We are running 20MHz XTAL) =5MHz Time Period = 0.2uS

Prescaller Period = 0.2 x 256 = 51.2uS

Overflow Period = 51.2 x 256 = 13107.2 uS = 0.0131072 sec

So we need 1/0.0131072

Over Flows to count for 1 sec = 76.2939 Overflows

TIMER - COUNTER

OPTION_REG:

PSA=0; // Prescaler is assigned to the Timer0 module

PS0=1; // Prescaler rate bits

PS1=1; // are set to “111”

PS2=1; // which means divide by 256

TOSE=0; // rising edge

TOCS=0; // Internal instruction cycle clock

TIMER - COUNTER

Another method to calculate the value of Timer:

The TIMER0 period could be calculated using this formula:

TIMER0 period = [(TMR0 + 1)] x 4 x Tosc x (TIMER0

prescaler value)

Ex: By selecting the TIMER0 prescaler of 2; PS2=0, PS1=0 and

PS0=0 bits in OPTION_REG register and Initial the TMR0

register value to 156 (99 more counts to reach its maximum

value of 255) with the system frequency clock of 8 Mhz

the PIC microcontroller TIMER0 overflow period can be

calculated as follow:

TIMER0 period = [((255 - 156) + 1)] x 4 x 1/8000000 x 2 =

0.0001s = 0.1 ms

TIMER - COUNTER

Sample Code

/* Init TIMER0: Period: Fosc/4 x Prescaler x TMR0

0.0005 ms x 2 * 100 = 0.1 ms */

OPTION = 0b00000000; // 1:2 Prescaller

TMR0 = 156 ; // Interupt every 0.1 ms

T0IE = 1; // Enable interrupt on TMR0 overflow

GIE = 1; // Global interrupt enable

………

// ISR (Interrupt Service Routine)

if(T0IF) { // TIMER0 Interrupt Flag

pulse_max++; // Pulse Max Increment

pulse_top++; // Pulse Top Increment

if (pulse_max >= MAX_VALUE) {

pulse_max=0;

pulse_top=0;

RC2=0; // Turn Off RC2

}

if (pulse_top == top_value) {

TMR0 = 156 ; // Initial Value for 0.1ms Interrupt

T0IF = 0; // Clear TIMER0 interrupt flag

TIMER - COUNTER

How to calculate the value of 156 or the value 99 of the TMR0?

TIMER0 period = [(TMR0 + 1)] x 4 x Tosc x (TIMER0 prescaler value)

 TMR0 = {TIMER0 period/(4 x Tosc x TIMER0 prescaler value)} – 1

 TMR0 = {0.1ms / (4 x (1/8MHz)) x 2)} – 1 = 99

 TMR0 = {0.0001s / (4 x (1/8000000) x 2)} - 1= 99 (Count value)

The value input to the TMR0 register =

= Max timer value – Count = 255 – 99 = 156

TIMER - COUNTER

If using INTERNAL crystal as clock:

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

(256 - TMR0) - The number of times in the timer will count

based on the register TMR0

TIMER - COUNTER

Example:

Suppose we want to create a delay of 0.5 second using

Timer0 What is the value of Count?

Calculation:

First, let’s assume that the frequency division by the Prescaler will be 1:256 Second, let’s set TMR0=0 Thus:

TIMER - COUNTER

If using EXTERNAL crystal as clock:

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

Example:

TIMER - COUNTER

Example: Delay 1 second

TIMER - COUNTER

Exercise:

1 Using PIC16f877, the oscillator is 20MHz, write the program to create the square pulse on pin A1 with the duty cycle is 50% and the frequency is 5KHz

2 Using PIC16f877, the oscillator is 20MHz, write the program to create the square pulse on pin A1 with the duty cycle is 70% and the frequency is 10KHz

TIMER - COUNTER

Timer 8 bits – Timer 16 bits

What is the differences between them?