From: Jonathan Kirwan
Subject: Re: Problems with cristal as clock for atmel AT90S8535
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NNTP-Posting-Date: Thu, 07 Nov 2002 00:03:15 GMT
Organization: AT&T Broadband
Date: Thu, 07 Nov 2002 00:03:15 GMT
On Wed, 6 Nov 2002 23:03:08 +0100, "madcat"
>I'm building a simple ISP flash board for an atmel AT90S8535.
>But i have some problems with the clock.
>At the moment i have a 8 Mhz cristal connected to X1 and X2 and two 22pF
>here a little ascii art to make it more clear:
> X2 |-------|-------][---|
> | cristal 8Mhz |
> X1 |-------|-------][---|
>-----| 22pF |
>The problem is that i can not get any signal out of it.
>But when i give a pulse via the parralellport or via a wave modulator
>(between 1Mhz and 8Mhz) the board works fine.
>I was wondering what i could do wrong here.
I've had problems with the AT90S chips, as well. Mostly with
the AT90S2313, but once or twice with the AT90S8515. Never once
with other microcontrollers, such as 8051's from various makers,
PIC, H8's, and so on, but with these I have had some trouble.
Using an external oscillator always works perfectly and it was
how I verified things were just fine with the systems, no matter
their complexity. It was only when I tried doing it with an
external crystal and carefully followed what I read to be their
instructions, that I ran into trouble sometimes. (I think it's
related to the power supply details, such as rise time, but I
haven't bothered to look more deeply as I've moved to other
controllers I like better, anyway.)
Imbalancing the caps helps a lot. Try one at 33pF and one at
15pF, for example.