8051 does not oscillate

Hi 8051 experts there

I have made a circuit for remote switching of or on a motor using

89C2051 and the program has checked out in the emulator. Also I have put the whole thing on a breadboard a hundred times and it has worked every time as expected without any flaw.

But when I made PCB, soldered all the parts and power it on it does not work - the crystal fails to oscillate. I have in fact taken the same components that form the crystal circuit after successful test to the PCB it still fails.

I have checked the PCB for any errors and also discontinuity for any breaks - I could find none.

And the crystal is just a slow one, 32.768 kHz clock crystal, that works without a hitch on the breadboard.

This is just baffling me. Can any of you suggest what else I should look for?

Thanks for your time and attention

ClueLess

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

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Tim Wescott
Control systems and communications consulting
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Reply to
Tim Wescott

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.

Reply to
Mike Silva

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

5V. so whenever there is a mismatch, try get the required voltage at the node by tracing the problem from its output........... so u have solved it.............
Reply to
mahfooz.sheikh

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

Reply to
mahfooz.sheikh

You need to add capacitance.!

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Reply to
Jamie

What value caps are you using around the crystal ?

Graham

Reply to
Eeyore

Unlikely IME.

Graham

Reply to
Eeyore

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?

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

Allen

Reply to
Allen Bong

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.

32.786 crystal, according to literature, needs 11 pf capacitors but I found this did not work and I had to use either 22 or 33 pf capacitors to get it work on the breadboard.

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

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Reply to
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

Reply to
Rich Grise

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

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Reply to
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

22pF cap to ground, has an _effective_ capacitance of 11pF. If this makes no sense to you, say so and someone will explain.
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Tim Wescott
Control systems and communications consulting
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Reply to
Tim Wescott

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