Oscilator input

[Shack]

Update: I have confirmed on another board that I can see the oscillator on Pins 9/10. So, this one has a problem which is quite suspicious.

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On a 28x part with an external crystal should you be able to see this with a scope on pin 9 or 10?

My chip is acting strange. I won't communicate with another chip. I see the clock pin go high a couple of times then just die so I am curious if my crystal is bad. I can not see any activity on pins 9 or 10 that the external crystal is connected to.

 

[Technical]

The ocsilloscope probe may well affect the crystal operation.

Can you download a program - if so the crystal is working!!!

 

[Shack]

I don't have a download circuit on this board so I can't test that.

I have a good scope and a good probe .... so I should be able to see this then?

 

[womai]

I never had problems observing the oscillator signal with an oscilloscope, even at 16 MHz. But of course the scope probe adds a few pF of parasitic capacitance, so Technical is right that probing _may_ affect operation. Typical number is 15pF for a cheap 60 MHz bandwidth probe. One thing though - make sure the probe is set to 1:10 or 1:20 divider mode, NOT 1:1 (which would load the resonator with the full cable capacitance and most likely stop any oscillation). Also, use as short a ground lead as possible.

Wolfgang

 

[Shack]

I never had problems observing the oscillator signal with an oscilloscope, even at 16 MHz. But of course the scope probe adds a few pF of parasitic capacitance, so Technical is right that probing _may_ affect operation. Typical number is 15pF for a cheap 60 MHz bandwidth probe. One thing though - make sure the probe is set to 1:10 or 1:20 divider mode, NOT 1:1 (which would load the resonator with the full cable capacitance and most likely stop any oscillation). Also, use as short a ground lead as possible.

Wolfgang

Thanks for the more detailed reply. I have a decent digital storage scope and a good probe set to x10.

When I power the chip on the signal is there and very clear. About 1 second later it goes flat so it looks like it stops. However, the Picaxe chip works in another board (no crystal) fine. I have the proper size caps across the crystal as well. I am really stumped as to why it stops. There are no other external connections to it and two different 28x parts do the same thing???

 

[Technical]

Have you got the PICAXE reset pull-up resistor and the PICAXE serin download pin tied low?

 

[Shack]

Have you got the PICAXE reset pull-up resistor and the PICAXE serin download pin tied low?

Yes I do. In fact I have 3 identical boards (proto PCB's)with nothing else connected to the Picaxe other than the xtal, (2) 20p caps and the 4K7 pull-up. Electrically the boards look fine.

1 board works fine
2 other boards ... oscillator starts, runs about 10 seconds then stops.

All 3 Picaxe parts work in the one board
No Picaxe parts work in the other two boards.

Obviously it is a PCB problem .... but I haven't a clue why since they are identical.

 

[andrew_qld]

Perhaps there s some stray inductance or capacitance on the board?

You could change the value of or remove the capactors.

Have you tried changing the working xtal into a non-working board to see if its the boards or the crystals?

Are the crystals socketted? Sometimes sockets play up with crystals and resonators.

 

[SD2100]

You've probably done it but maybe check for bad solder joins and hairline breaks in the tracks, I've been caught before with scratched transparencies when making PCB's, especially tracks under .010" wide.