Start up problem

alchazz · Joined: 03 Oct 2006 Posts: 13

I have a bunch of 16F84s and 16F84As and am trying to use them up. I've run into a problem in that I can't get them to work. It seems that they don't oscillate. I have used numerous ceramic resonators and crystals with no luck.

I was able to program them using version PCM 3.059. Now that I have PCW v3.190 they won't work. Is there something haywire with PCW v3.190?

I program 12C508As and 16F877As with it with no trouble. Do I have to load my old PCM to get this to work? If I do, will it interfere with my new PCW version?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I don't have PCW, but I installed the command line PCM compiler vs.
3.190 and compiled the following program. I programmed it into a
16F84A on a PicDem2-Plus board and it works OK. It blinks the LED
connected to Pin B0. I think your PCW vs. 3.190 should work.

Guest · Posted: Wed Oct 04, 2006 8:07 am

This is the code I used as a test.

#include <16F84.h>
#use delay(clock=4000000)
#fuses XT,NOWDT,NOPROTECT,PUT
#define red_led PIN_B0
#define green_led PIN_B1

void main()
{

while(true)
{
output_low(red_led);
output_high(green_led);
delay_ms(500);
output_high(red_led);
output_low(green_led);
}

}

The red led would illuminate and the green led would not, as expected, and nothing else would happen. I checked the outputs of the two pins driving the leds and the voltages were correct.

alchazz · Joined: 03 Oct 2006 Posts: 13

500 mS after my previous post, I noticed the error.

I reprogrammed and then tried the circuit. And still no go.

Then.....as I moved my hand close to the breadboard, the thing started to work! I moved my hand away, and it stopped. Hmmm.

So, I rearranged the leads more carefully on the breadboard and now it works every time.

Arrggghh!

alchazz · Joined: 03 Oct 2006 Posts: 13

Wow, looks like a lot of people are looking at my thread.

Are you all having start up problems with your programs and/or devices?

Smile

dyeatman · Joined: 06 Sep 2003 Posts: 1933 Location: Norman, OK

When I used resonators/crystals very early on, my rule was always to keep the crystal/caps as close to the chip as possible. Never have leads from the chip to the crystal longer than 1/4-1/2 inch. This is especially true as you go up in frequency.

I now buy oscillator half cans for a buck a piece. These always start, I get guaranteed freq accuracy and don't have to worry about caps or circuit influence. However, I still always use good high frequency layout rules. One advantage, if you socket the can the frequency can be easily changed any time you wish.

bill147 · Joined: 26 Oct 2004 Posts: 13

Some years ago I had this same problem. There was a note on the Microchip web site that said when using a crystal the 2 bypass capacitors should not be the same value. For example if you use a 18pf on osc in and a 22pf on osc out then you will not have a startup problem.

Bill