Robert Luketic

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11 years, 2 months ago.

Unable to get any oscillation to clock LPC1768

I am unable get any oscillation out of the oscillator. I have been trying to get the 12.000MHz from a18pF SMD Crystal (MPN: ABM3B-12.000MHZ-B2-T) but unsuccessful. I scope the signal and I am seeing no oscillation at all. Attached is my schematics and layout. Can anybody tell me what went wrong?

Here is the the info of the Oscillator. http://www.digikey.ca/product-detail/en/ABM3B-12.000MHZ-B2-T/535-9116-1-ND/675633

/media/uploads/bjsrlu/lpc1768.pdf

2 Answers

Daniel Blanch
9 years, 10 months ago.

Hi, did you find the error? I have the same just now.

Thanks

Daniel

Andy A
9 years, 10 months ago.

Try dropping the capacitor values a little, that will sometimes help with crystal startup issues.

In the original post the schematic and component placement looks ok however the routing leaves a lot to be desired, the power and ground traces are a spiderweb of connections all over the place. It's hard to say if poor quality noisy ground connections are to blame but they certainly don't help. You want short, low impedance power and ground connections, not normal width signal traces that snake their way around the board.

Also putting a scope on the signal isn't much use. The crystal is designed for a load of 18pF. A scope probe adds another 15pF to the trace, as soon as you probe it you've got almost double the design capacitance on there and it has no chance of starting.