8051 does not oscillate

Did you autoroute, or hand route without paying attention to stray impedances?

If your wiring and uC oscillator configuration correct then my guess is that either you've installed a damaged part, or perhaps the breadboard layout had extra capacitance that your PCB does not have, and that your oscillator needs to start up.

the best way to debug a circuit, be it analog or digital, is to identify few important nodes and verfiy their voltage values (both calculated and observed values must be equal). but digital circuits are easier to follow coz voltage value is either 0 or somewhere near

a mere multimeter will suffice to debug the problem..........

You need to add capacitance.!

What value caps are you using around the crystal ?

Graham

Unlikely IME.

Graham

As others have pointed out it could be your layout. Best is to post the layout and if you can, also the schematic.

Do you have a full ground plane on that board?

Try with a higher freq xtal say 6MHz and see it oscillates. If it does, then you need to add capacitance.

Allen

It is really gratifying to see the number of responses to my query - the suggestions have really given me a number of leads to test further.

I removed the crystal and capacitors from the pcb and connected pins X1 and X2 of 8051 direct to the breadboard - it works.

On the breadboard everything is widely spaced but on the pcb they are very close. So it could be a layout problem. I have redesigned the pcb isolating the crystal circuit from the other parts, added more space between pads and also added a ground plane under the crystal parts. I will get the board made and come back after trying it.

And thanks again to all of you for your time and attention

Thanks for your time and attention

ClueLess

I hate to nag, but you _did_ include the caps in your layout, didn't you? If you've got, say, 2x 22 pf and it still doesn't start, you might try changing the ratio of capacitances (as long as the series connection still comes out at 11 pf) to diddle with the drive and feedback.

Good Luck! Rich

Hi Rich

That was the first thing I did, put two 22s in series to get 11 pf and try. All data on 32.786 kHz crystals said 11pf but the dozen crystals I had insisted on either 22 or 33 pf

I feel that it is the layout, I am just waiting for the new board, expected Friday when I will stuff it and see where it goes.

Thanks for your time

Thanks for your time and attention

ClueLess

Are you putting 11pF on _each side_ of the crystal? If so, you're using half of what you should.

The normal circuit, with a 22pF cap to ground, a crystal, and another