# Seemingly unstable basic PIC18F2550 circuit

From my work I've had quite a bit of experience with hardware development, but purely from a supervisory role, and so recently I've been playing around with ground up MCU circuit design to try to get a better understanding.

I put the following circuit together to allow me to play around with the MCU registers a bit and it does work - only intermittently.

The LED blinks as intended, then stops for an arbitrary time, flashes again a different number of times, off again etc. There doesn't seem to be any cyclic behaviour to it. It starts working without any external input (i.e. nudging it) so doesn't seem like a loose connection either. I realise the second Vss pin isn't grounded in the schematic, but this didn't help the circuit either when I tried it. Could it be because Vusb isn't grounded? I would have thought this would only affect USB operation.

Code:
Code:
#include <stdio.h>
#include <stdlib.h>

#include <p18f2550.h>
#include <delays.h>

#pragma config FOSC = INTOSCIO_EC

#pragma config WDT = OFF

void main() {

TRISAbits.TRISA1    = 0;    // Set RA1 as output
LATAbits.LATA1      = 1;    // Set RA1 as HIGH

while (1)
{
LATAbits.LATA1 = ~LATAbits.LATA1;   // Toggle LED pin
Delay10KTCYx(25);                   // Delay
}
}

lewando
Homework Helper
Gold Member
Ceci n'est pas une pipe

Why wouldn't you ground pin 8 (VSS)?

: Oh I see you tried that. Did you really ground that?
: VUSB should not be grounded, it is an output.
: Have you tried stepping through the code with a debugger?

Last edited:
Averagesupernova