External oscillator problem

Question

I have this sample program to blink LED with PIC18F4550 using external oscillator.

I'm using a 20.000 MHz crystal oscillator with 30pF capacitors.

The problem is when I wire the capacitors the the ground the LED doesn't blink (still off), but when I disconnect them from ground (not wired) the LED starts blinking. Blink speed change with _XTAL_FREQ.

Program works well with internal oscillator (just by setting FOSC = INTOSCIO_EC)

Is there direction wiring of capacitor or oscillator?

Ground here is PIC Vss

This is when using PICKIT2 directly. When using the circuit alone like the scheme, nothing happens with FOSC = INTOSCIO_EC configuration (not working), but works well with internal oscillator.

#include<p18f4550.h>
#include "delays.h"

/* COMPILER DIRECTIVES FOR CHIP CONFIGURATION BITS*/

#pragma config FOSC = HS //INTOSCIO_EC    /* Sets the oscillator mode to HS */
#pragma config WDT = OFF   /* Turns the watchdog timer off */
#pragma config LVP = OFF   /* Turns low voltage programming off */
#pragma config DEBUG = OFF /* Compiles without extra debug code */

#define _XTAL_FREQ   12000000  //delay_ms change by changing this


void main(void) {
    TRISD = 0xF0; // PORT B Setting: Set all the pins in port B to Output.
    while (1) {

        LATDbits.LATD0 = 1; // RD-0 to High
        LATDbits.LATD1 = 1; // RD-1 to High

        __delay_ms(50); // Half sec delay

        LATDbits.LATD0 = 0; // RD-0 to LOW
        LATDbits.LATD1 = 0; // RD-1 to LOW

        __delay_ms(50); // Half sec delay
    }
}
/* THE END */

Answer 1

OK, the picture tells the big story - it's a layout problem - the xtal needs to be mounted right up at the pic pins and the two capacitors need to connect to 0V via the shortest possible route. At the moment, the wires to the xtal and caps has too much inductance and not surprising this will not work correctly.

Answer 2

Just in case Andy's answer doesn't solve the problem, here's something else I see. You've got _XTAL_FREQ set to 12MHz with a 20MHz crystal.

Plus, your delay functions are delaying for 50ms, not 500ms like your comment indicates. With the mismatched _XTAL_FREQ value and the mis-typed delay time, the actual delay will be (12/20)*50ms = 30ms.

That's 33Hz. The human eye may not be able to tell it's blinking at that rate.

Answer 3

Try these configuration bits:

#pragma config PLLDIV   = 5         /* PICs 96 MHz PLL prescaler divided 
                                       by 5 for 20MHz input */
#pragma config CPUDIV   = OSC1_PLL2 /* System clock post scaler 
                                       [OSC1/OSC2 Src: /1][96MHz PLL Src: /2] */
#pragma config FOSC     = HSPLL_HS  /* HS oscillator, PLL enabled */

You will end up having clock frequency of 96/2MHZ = 48MHz which will produce instruction frequency of 12MHz which is the maximum for PIC18F4550.

Also PIC18F4550 datasheet http://ww1.microchip.com/downloads/en/devicedoc/39632c.pdf on page 27 recommends using 15pF capacitors for 20MHz crystal.

Looking at your picture I'm pretty sure you have a small typo in there. 2 of oscillators wires are connected wrong way

Answer 4

after ensuring that your hardware connections are fine as pointed out, the other error must have been your capacitors value. I had exactly the same error where the MCU would blink the LED without the capacitors but turned off immediately after plugging the capacitors. The capacitors value I used were "223pF" instead of "22pF". Ceramic capacitors value are always in picofarads and therefore you have to do the correct calculations. "223 = 22000pf" while "0.022nF = 0.022x1000pF = 22pF." hope it helped you.