Thread: Timer0 RTC

I am currently using an external 32khz crystal to act as a timer for my pic18f45k20. I have made some progress with help from some on this forum but I believe some of my calculations may be wrong.

The code:

Code:

/** C O N F I G U R A T I O N   B I T S ******************************/


#pragma config FOSC = LP, FCMEN = OFF, IESO = OFF                       
#pragma config PWRT = OFF, BOREN = SBORDIS, BORV = 30                       
#pragma config WDTEN = OFF, WDTPS = 32768                                    
#pragma config MCLRE = OFF, LPT1OSC = OFF, PBADEN = ON, CCP2MX = PORTC 
#pragma config STVREN = ON, LVP = OFF, XINST = OFF                          
#pragma config CP0 = OFF, CP1 = OFF, CP2 = OFF, CP3 = OFF                
#pragma config CPB = OFF, CPD = OFF                                
#pragma config WRT0 = OFF, WRT1 = OFF, WRT2 = OFF, WRT3 = OFF             
#pragma config WRTB = OFF, WRTC = OFF, WRTD = OFF                     
#pragma config EBTR0 = OFF, EBTR1 = OFF, EBTR2 = OFF, EBTR3 = OFF
#pragma config EBTRB = OFF                                        




/** I N C L U D E S **************************************************/
#include "p18f45k20.h"
#include "delay.h"
#include "delays.h"
#include "switch header.h"


/** D E C L A R A T I O N S ******************************************/












void main (void)
{


int Day =0; // number of days
int Counter =0; // counter
int Sec = 0; // seconds passed




PORTA = 0x03;            //Preset RA1,RA0 to 1    
TRISA = 0x00;            




INTCON2bits.RBPU = 0;        // enable PORTB internal pullups
WPUBbits.WPUB0 = 1;            // enable pull up on RB0
ANSELH = 0x00;              // AN8-12 are digital inputs (AN12 on RB0)
TRISBbits.TRISB0 = 1;       // PORTB bit 0 (connected to switch) is input (1)


    // Init Timer
INTCONbits.TMR0IF = 0;          // clear roll-over interrupt flag
T0CONbits.T08BIT = 0;           // 16bit operation - reduces the need to use prescale 16   
T0CONbits.PSA = 1;           // WDT enabled prescale 1:1 
TMR0H = 0xE0;                      // clear timer - always write upper byte first
TMR0L = 0xBF;
T0CONbits.TMR0ON = 1;           // start timer


/*Fosc = 32KHz
Fcycle = Fosc/4 = 32KHz/4 = 8KHz
Tcycle = 1/Fcycle = 125us
Cycle count (no prescaler) for one second = 1s/125us = 8000
16bit overflow value = 0xFFFF (65535 in dec)


Calculate TMR0 (H&L) values:


65535-8000 = 57535 (0xE0BF)
*/




while (1)
{




if(INTCONbits.TMR0IF)        // when overflow occurs increment sec integer
{
Sec ++;


}


if (Sec==10) // 10 seconds represent one day for now 
{
Day ++; // in debug mode I am running cursor here it takes ages to return a value of one for days?
Sec = 0;
}

The code appears to work, however my timer does not appear to be overflowing every 1sec? Is there a problem with the calculation?

...In fact when setting the prescale to WDT to give me a prescale of 1:1 the value of days never increments.