[PIC18F4550] Why Timer0 is not giving correct value

[milan.rajik]

I am attaching my mikroC PRO PIC project files. I have to get 1 us timer0 interrupt. RD0 should toggle once every 1 us but it is toggling once every 0.5 us.

The oscillator configuration is like this.

Datasheet page no. 24

Primary oscillator PPLDIV/5 = 20 MHz / 5 = 4 MHz

USB Clock = 96 MHz / 2 = 48 MHz

CPUDIV/2 HSPLL = 96 MHz / 2 = 48 MHz

Primary Oscillator

OSCCON.SCS0 = 0;
CSCCON.SCS1 = 0;

I have used mikroE Timer calculator to generate Timer0 Interrupt code.

 

[Vbase]

In 1us the uC can do the most 12 instructions, you have more than 12 in your loop, the 0.5us you get is an illusion.

 

[milan.rajik]

This is the asm code of the ISR. The instructions are less than 12.

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Code ASM - [expand]
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Interrupt:
 
;timer_interrupt.c,13 ::                 void Interrupt() {
;timer_interrupt.c,14 ::                 if (TMR0IF_bit){
        BTFSS       TMR0IF_bit+0, BitPos(TMR0IF_bit+0) 
        GOTO        L_Interrupt0
;timer_interrupt.c,15 ::                 TMR0IF_bit = 0;
        BCF         TMR0IF_bit+0, BitPos(TMR0IF_bit+0) 
;timer_interrupt.c,16 ::                 TMR0H = 0xFF;
        MOVLW       255
        MOVWF       TMR0H+0 
;timer_interrupt.c,17 ::                 TMR0L = 0xF4;
        MOVLW       244
        MOVWF       TMR0L+0 
;timer_interrupt.c,19 ::                 LATD.F0 = ~LATD.F0;
        BTG         LATD+0, 0 
;timer_interrupt.c,20 ::                 }
L_Interrupt0:
;timer_interrupt.c,21 ::                 }
L_end_Interrupt:
L__Interrupt5:
        RETFIE      1
; end of _Interrupt

---

 

[Yosmany325]

Hello milan.rajik, maybe your asm listing (I guess is provided by MikroE) does not include context saving code associated with the interrupt. I'm not sure how this compiler works but mostly all C compilers do context saving prior to executing user's interrupt code.

 

[bigdogguru]

I have to get 1 us timer0 interrupt. RD0 should toggle once every 1 us but it is toggling once every 0.5 us.

How are you measuring the output pin frequency?

 

[milan.rajik]

I am measuring output freq using an Oscilloscope. Also I am testing in Proteus. Both giving same result. RD0 is toggling once every 0.5 us.

 

[Vbase]

Are you counting BTFSS as 3 and GOTO as 2 cycles. What about all the code in the main function, why dont you count that?
If the output toggles it doesn't mean that the uC is doing what you want it to do. Try 10us.