Timer0 issue with PIC18F4550 @ Fosc=32M

Hello all

-I'm trying to build some small projects to test timers and oscillator settings

- My project now is very simple , just toggeling LED status every 1s
- Issue is Timer0 is not operated correctly , sometimes LED is kept on , sometimes kept off and
sometimes LED is blinking at arbitary rate
- My Hardware is very simple, just LED connected to RB0 and Crystal 4M connected
- I'm using my Fosc from PLL , 4M connected to drive PLL , and PLLPostscalar /3 is used
- So based on that , Fosc that will be input to TMR0 is 32M (96/4)
- This is my Code :-

####################################################################################################


#include <htc.h>


// PIC 18F4550 fuse configuration:

// Config word 1 (Oscillator configuration)


__CONFIG(1, USBPLL & IESODIS & FCMDIS & HSPLL & CPUDIV2 & PLLDIV1);


// 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);


int Amr;


main()
{
PORTA = 0x00;
PORTB = 0x00;
TRISB = 0x00;

IPEN = 0;
T0CON = 0b11000100;
INTCON = 0b11100000;
INTCON2 = 0b11110000;

Amr = 0;
TMR0L = 5;


while(1)
{

if (Amr == (1000) )
{
Amr = 0;
PORTB =~ PORTB ;
}
}
}



// LOW priority interrupt
void interrupt low_priority lowISR()
{

TMR0L = 5;
TMR0IF = 0;
Amr = Amr + 1;


} // end of LO interrupt

####################################################################################################

- The Result that LED is not blinking correctly as expected ,it seems Timer0 is not stable, and i don't know reason
- When I tried to change the PLLPOSTscalar to /4 ( Fosc = 24M ) and adjust variables ( Amr to 750) to
sustain 1s period , issue was completely disappear !
- so I Just need to know , does TMR0 for PIC18F4550 has Fosc limits , do i have to follow any guidlines when uc Fosc
reaches high frequency such Fosc 32M ?
- Is there's any hardware practices i should follow to eliminate issue
- Is there's any errors in my Code ?
- Thanks to help me regarding this issue

Do you have the two capacitors that are required for the crystal?

Mike.

Pommie said:

Do you have the two capacitors that are required for the crystal?

Mike.

Click to expand...

Hello Mike , Thanks for reply

Actually i put crystal connected directly to PIC , I don't use any capacitors

So , Are capacitors important when Fosc reach 32M , as I used not to connect them when operating at lower Fosc

Can u please advise me which capacitors values i should use ?

Thanks

Its the quality of the 4M crystal aswell.. I always put 22pf on each leg to ground some folk even put a 1meg resistor for a tad more stability..

Look at Figure 2-2 of your datasheet. They give you a recommended circuit for a crystal that shows the capacitors.

Hello Ian Rogers & Rusttree

Thank u for reply

I will try your suggestion and will get back to you to tell results

Thanks in advance

Solved !

Hello all

I found the issue , I have added 200n Cap between supply rails and Timer0 operation was stable , issue is solved finally !

Thank u everybody for advices and hints , have a nice day !