Propeller external crystal help please?

Robofreak

Hello all,

I've recently built my 2nd programming board for the Prop, with the goal of being more flexible than the last. And so, I've built it so that an external crystal can be inserted/removed via a 3-pin female pcb connector (2 pins tied together going to one of the osc inputs, and the other single going to the 2nd osc input). I have some questions about this setup, though... and remember, I don't know much of anything about using crystals haha

1) Will the connector and solder joints between the external crystal and prop significantly change the frequency of the crystal as the prop sees it?
2) What is an "acceptable" distance from the prop for the crystal to be, under my described circumstances?
3) What is, most likely, the fastest "low-speed" crystal I can connect to the prop programmed to run with PLL16X?

All help is appreciated! [noparse]:)[/noparse]

- Robofreak

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Austin Bowen,


"One must watch out for mechanics. They start out with a sewing machine, and end up with the atomic bomb" - A quote from someone that I saw on Addall.com

"Not only do I see the glass as half empty, I wonder if anyone poisoned the glass with arsenic" - My dad, inside joke haha

Duane Degn

Robofreak,

I'm no crystal expert but I'd bet you'll be okay with your socket.

Have you used a Prop Proto Board?· It has a crystal socket on the board so you can exchange crystals.· (I think the Demo board also has the sockets, but I loaned mine out so I can't check.· The Pro.·Dev. Board also uses the sockets.)

There was mention of where to buy the sockets on the forum.· You could try an advanced search through Google and try to find it.

I've read several times that traces to the crystal should be kept short, but I don't know specifics.

Duane

Leon

1 cm should be OK.

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Leon Heller
Amateur radio callsign: G1HSM

Robofreak

Well, the thing is, I put in a 6.000MHz crystal that I bought to test the setup, and when I tested serial communication between the prop (with XTAL1 + PLL16X set) @ 9600bps, I had to change the "_clkfreq" constant to an avg. of 7_375_000 Hz. I'm sitting here wondering what the heck could make a 6.000MHz crystal run the prop at 7.375MHz... but it works. The thing is, as far as I'm concerned, it shouldn't and I have no idea why! surely crystals aren't manufactured with a 23% error rate haha

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Austin Bowen,


"One must watch out for mechanics. They start out with a sewing machine, and end up with the atomic bomb" - A quote from someone that I saw on Addall.com

"Not only do I see the glass as half empty, I wonder if anyone poisoned the glass with arsenic" - My dad, inside joke haha

Leon

When overclocking the Propeller like that, some people have found that the crystal tracks need to be very short, and you need to pay a lot more attention to decoupling and PCB layout. The crystal might be operating in a different mode, changing the frequency, although that would be very unusual. Try a standard 5 MHz crystal and see what happens.

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Leon Heller
Amateur radio callsign: G1HSM

Post Edited (Leon) : 7/31/2010 2:04:49 AM GMT

Robofreak

@leon: The 5MHz crystal works perfectly fine. hmm..

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Austin Bowen,


"One must watch out for mechanics. They start out with a sewing machine, and end up with the atomic bomb" - A quote from someone that I saw on Addall.com

"Not only do I see the glass as half empty, I wonder if anyone poisoned the glass with arsenic" - My dad, inside joke haha