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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Problems with cristal as clock for atmel AT90S8535
Date: Thu, 07 Nov 2002 01:38:39 GMT
NNTP-Posting-Date: Wed, 06 Nov 2002 17:38:39 PST
> 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:
> -----| 22pF
> 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 used a hosonic 8Mhz b214 in a HC-49/U case.
> (it's a cristal in a metal case with two pins)
> I can not find any datasheet of it.
> I tried to measure the cristal via a osciloscope.. but i think the signal is
> to weak to measure.
> many schematics i see on internet i see have this construction.
> so i assume the atmel has an amplifier build in for the cristal.
> I thougt the two small capacitors were there to make teh cristal osciliate
> out of it self and it did not need any extra signal to make a signal.
> How does a cristal actualy work.. does it just make a sinus wave all the
> time out of itself?
> or does it need any trigger?
> Is there anyone here who might be able to help me with this problem?
> Who made a simple flash board and could give me some advice?
> Thanks in advanced
The oscillator that Atmel uses is simply an inverter with a poly
resistor between the input and the output. The resistor is a layout
option and I have seen it omitted in some products. With the chip
powered and nothing connected to the pins, the output should be sitting
at about the CMOS threshold for the technology or it may oscillate from
stray capacitance. If either, everything is fine. If the output is at
one rail or the other, either the resistor was omitted or there is
leakage to ground or Vdd likely outside the chip. If the internal
resistor was omitted, connecting a 10meg resistor between the pins
should bring the output to input threshold and the oscillator will work.
When checking, only probe the output because the oscillator may fail to
function if there is any significant load on the input. (Most of the
Atmel oscillators will tolerate a 10meg oscilloscope probe on the input
but don't depend on it.)
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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