Can an 8515-8 run at 8.192mhz?

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I happen to have a big bag of 8.192MhZ crystals, which I've used quite a bit with 2313-10's.

I also have a bunch of 8515-8 chips. Rather than buy a bunch of 8.00MhZ crystals, I'm tempted to just build my projects with the crystals I have. THe obvious problem being that these crystals are just "a wee bit" faster than the processor is rated for.

Anyone have any experience with this? I realize that Atmel cannot officially endorse such a use, but I suspect there's enough fudge factor that it would work just fine. (It did seem to work just fine when I tested it briefly, but of course the risk would be that it stops working at some temperature level, supply voltage deviation, or whatever other condition that I didn't test).

Anyone have any advice or experience to offer?

Thanks

Erik

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Hi Erik

I would say: no problem. When AVR's first came out, Atmel actually spec'ed some of them at 20MHz (eg 1200)! Then some problems were found writing to the EEPROM and they downgraded the chips to various values. The core will still happily run at 20MHz, but at that speed you will most likely experience some timing problems (eg EEPROM).

But 8.192MHz shouldn't be a problem even with using EEPROM (it's only a 2.4% difference).

Try a 12Mhz or 16Mhz crystal and see how it flies!!

If you're designing for commercial applications however, I suggest you stay within the spec's. If anything ever goes wrong, you'll have no leg to stand on if you've designed 'out of spec'.

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Hi Erik,

as I said in one of the earlier messages in this forum, I suspect that the speed ratings are arbitrarily issued after selection testing.

Look at the 8515 datasheet for example and you will notice speed ratings well beyond 8 MHz.

I remember reading that Intel used to label their processors with speed ratings according to the results of a failure test at different speeds. It must be true to infinitely small, but nevertheless present, manufacturing tolerances.

Interesting to hear that Atmel downgraded their chips, too.

I am absolutely sure that 8MHz labelled chips will work no problem with your quartz.

-aquataur

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Just specify you minimum supply voltage to be away from the very bottom end of the part specification range (by a quarter of a volt maybe) and you will be just fine with this tiny hike of the clock. As implied above you will probably be fine anyway - but that's another story, and good designers don't run close to the wind.

Terry

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