Mastri

Posted on March 4, 2011 by Dhinesh
1
Hello Guys;
Happy to see you back again, to check how to program timer.
We need to do some mathematical calculation to configure the timer. Lets see how.
Clock Source : 20MHZ.
so FCPU = Clock/4                           (20Mhz/4)
FCUP = 5MHZ.
Time Period= 1/FCPU                   (1/5MHZ)
Time Period = 0.2uS.
Prescaler = 256.
Prescaler Period =0.2us * 256
Prescaler Period= 51.2us
Overflow Period = 51.2us * 256 = 13107.2us
Overflow Period = 0.013107 S
For 1 Sec time delay we need 1/0.013107
1 Sec = 76.29 Overflow.
The best way to understand the working of timer is Led Blinking. We are going to program to blink a led every 1Sec (76 Overflow).
Registers Configuration:
T0CON is Timer0 register. we need to configure it according to ourcalculation.
Prescaler: Bit 0 to 2 are prescaler selection. we need to configure according to table.
T0PS0=1; //Prescaler is divide by 256
T0PS1=1;
T0PS2=1;
PSA: we need to low  this bit, so that it will take prescaler select bit.
PSA=0;  //Timer Clock Source is from Prescaler
T0CS: Timer source select bit.
T0CS=0;     //Prescaler gets clock from FCPU (5MHz)
T08BIT: To make timer work in8 Bit mode or 16 Bit mode.
T08BIT=1;   //8 BIT MODE
TMR0ON: Timer0 ON/OFF Bit
TMR0ON=1;      //Now start the timer!

Now we need to configure the Timer interrupt,
Timer0 Interrupt:
TMR0IE=1;   //Enable TIMER0 Interrupt
Peripheral Interrupt:
PEIE=1;     //Enable Peripheral Interrupt
Global Interrupt:
GIE=1;      //Enable INTs globally
We have successfully configured the timer. letsbuilt and test drive it.
SOURCE CODE:
#include <htc.h>
// PIC 18F4550 fuse configuration:
// Config word 1 (Oscillator configuration)
// 20Mhz crystal input scaled to 48Mhz and configured for USB operation
__CONFIG(1, USBPLL & IESODIS & FCMDIS & HSPLL & CPUDIV1 & PLLDIV5);
// Config word 2
__CONFIG(2, VREGEN & PWRTDIS & BOREN & BORV20 & WDTDIS &WDTPS32K);
// Config word 3
__CONFIG(3, PBDIGITAL & LPT1DIS & MCLREN);
// Config word 4
__CONFIG(4, XINSTDIS & STVREN & LVPDIS & ICPORTDIS & DEBUGDIS);
// Config word 5, 6 and 7 (protection configuration)
__CONFIG(5, UNPROTECT);
__CONFIG(6, UNPROTECT);
__CONFIG(7, UNPROTECT);
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 FCPU (5MHz)
T08BIT=1;   //8 BIT MODE
TMR0IE=1;   //Enable TIMER0 Interrupt
PEIE=1;     //Enable Peripheral Interrupt
GIE=1;      //Enable INTs globally
TMR0ON=1;      //Now start the timer!
//Set RC1 as output because we have LED on itTRISC=TRISC&0C11111101;
while(1);
}
void interrupt ISR()
{
//Check if it is TMR0 Overflow ISR
if(TMR0IE && TMR0IF)
{
//TMR0 Overflow ISR
counter++;  //Increment Over Flow Counter
if(counter==76)
{
//Toggle RB1 (LED)
if(RC1==0)
RC1=1;
else
RC1=0;
counter=0;  //Reset Counter
}
//Clear Flag
TMR0IF=0;
}
}
You can download Project Here
Posted in PIC Tutorials | 1 Reply
TimerIn PIC18f4550
Posted on February 22, 2011 by Dhinesh
Reply
Welcome Back;
Sorry guys i was busy with my office work, currently developing device driver for USB. I will try to update my blog regularly.
This tutorial is all about timer in PIC18f4550. Timers are common features of all microcontroller. In simplified terms a timer is a register whose value keeps increasing or decreasing  by a constant ratewithout the help of the CPU.
The Timer register can have the following bit length
* 8 bit timers – These can count between between 0-255
* 16 bit timers – These can count between 0-65536
* 32 bit timers – These can count between 0-4294967296
Timers are also called Counters this is because they can be used to count external events.
This clock source for the timer is obtained from the CPU...