Hi,
I'm planning to do PWM and Timer2 with USB feature using PIC18F4550 with crystal of 20MHz.

What I intend to do is to have both PWM and Timer2 interrupt running on 20MHz as in FOSC = 20 MHz.

Attached is the print screen of my settings in configuration bits.

To test whether the code is running at 20MHz or not, i program a simple code to blink the LED ON for 1 second and then OFF for another second and i measure its time by using stopwatch. What i found is the code is running slower compared to the stopwatch.

Below is how i did the blinking code. I put it in a while loop so that the PIC will do nothing else except blinking the LED.

Code:

// I'm using 20MHz crystal oscillator.
void main(void)
{
    InitializeSystem();
    while(1)
    {
        USBTasks();         // USB Tasks
        ProcessIO();        // See user\user.c & .h
        while (1)
        {
                PORTD = 0xFF;	
		Delay10KTCYx(200);
		Delay10KTCYx(200);
		Delay10KTCYx(100);
		PORTD = 0x00;
		Delay10KTCYx(200);
		Delay10KTCYx(200);
		Delay10KTCYx(100); 
		}
    }//end while
}//end main

The reason why i did the above testing is to ensure that the PIC is indeed running at 20MHz so that when I do PWM and Timer2, I can use FOSC = 20MHz for the calculation. However, it seems that the PIC is running abit slower than 20MHz and because of that, I'm afraid when I do the PWM and Timer2, the result will be off.. Can somebody pls advice..
Thanks.

mvs sarma:
Yes, I did try the stopwatch and it indicates that instead of 1s, it takes about 1.2s which is about the same as the actual stopwatch.
Would it be safe to say that the extra 0.2s is the time it requires to call those functions and not because of the PIC not running on 20MHz..? If it is safe then if i do Timer2, will the result be accurate..?

TiagoSilva:
I place the blinking code in "void main(void)" function because i don't want any 'interruption' while i test the blinking of LED. At this point i don't need the USB feature yet. My whole idea is just to see how fast/slow the LED blink and how close/far it is from the 1s intended. However, when I actually do the code later, i will do it in ProcessIO.
BTW, there is an ebook in welcome - gigapedia.com on USB if you want. The title is "Advanced PIC Microcontroller Projects in C: From USB to RTOS with the PIC18F Series" ebook. Its free but you have to register as member. The book uses MikroC compiler tho.

Thanks all.

The timing suggest that your chip is running at 16MHz. The only way this can happen is if you have the PLL output divided by 6 as the clock source, but your config has HS selected. Have you tried setting it in source, I never use MPLAB to set the config as I find it confusing.

Mike.

Originally Posted by Pommie

The timing suggest that your chip is running at 16MHz. The only way this can happen is if you have the PLL output divided by 6 as the clock source, but your config has HS selected. Have you tried setting it in source, I never use MPLAB to set the config as I find it confusing.

Mike.

Hmm, do i set it at CONFIG1L and CONFIG1H register?
Is the code below correct..? By the way, I would like the FOSC to be 20 MHz and the USB at 96MHz if possible..

Code:

// USB clock source comes from the 96 MHz PLL divided by 2
// Primary oscillator used directly for system clock (no postscaler)
// Divide by 5 (20 MHz oscillator input)
CONFIG1L = 0b00100100;

// Oscillator Switchover mode disabled
// Fail-Safe Clock Monitor disabled
// HS oscillator (HS)
CONFIG1H = 0b00001100;

Thanks.

I always use the config directive. You can find a list of parameters in the MPLAB help file - it's so confusing they gave it its own help section.

I think for your application you want,

Code:

#pragma config WDT = OFF, LVP=OFF 
#pragma config FOSC = HS, PLLDIV = 5, CPUDIV = OSC1_PLL2, USBDIV = 2

Mike.

Oh.. thanks..

BTW, when i tried to enter the code below, it gives me compilation error. I also cannot seem to find CONFIG1L and CONFIG1H in PIC18F4550's header file. Just for understanding sake, is there any way to set the PIC registers by writing to it directly..?

Code:

// USB clock source comes from the 96 MHz PLL divided by 2
// Primary oscillator used directly for system clock (no postscaler)
// Divide by 5 (20 MHz oscillator input)
CONFIG1L = 0b00100100;

// Oscillator Switchover mode disabled
// Fail-Safe Clock Monitor disabled
// HS oscillator (HS)
CONFIG1H = 0b00001100;

Originally Posted by MrNobody

Just for understanding sake, is there any way to set the PIC registers by writing to it directly..?

Not that I am aware of. You can probably do it by using a section but I find it easier to just use the directive.

Mike.

TiagoSilva:
MPLAB interactive configuration..?
How do i use that..?
Where can i find it..?
Thanks..

Pommie:
Tried the directive. The result is the same as before.. So i guess the time difference compared to the stopwatch is because of the latency or extra time needed to call the delay function.. Will try timer2 as soon as i can and hope timer2 will give me a more accurate result.
Thanks anyway.

TiagoSilva,

I guess you missed the attachment to the first post.

Mike.