crystal

Archiver

hello all,

Finally got my i2c working, in combination with a timer chip. Only problem i
have, the chip uses a crystal, 32,768 khz. In serie there is a capacitor
5pf. But the timer chip gives a time that is much to slow, 1 sec is by my
chip about 5 secs, so to get a minute is for my chip 5minutes.

This means the crystal is 'too slow'. What i have figured out, that when i
put my hand near the crystal/capacitor the time is running much faster. Is
the crystal a kind of antenna? How can i make a good 32,768 khz? the
capacitor is a trimmer, but what i try, the time doesn't react at it.

thanks pieter-jan


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Archiver

>Finally got my i2c working, in combination with a timer chip. Only problem
i
>have, the chip uses a crystal, 32,768 khz. In serie there is a capacitor
>5pf. But the timer chip gives a time that is much to slow, 1 sec is by my
>chip about 5 secs, so to get a minute is for my chip 5minutes.

Hi pieter-jan,

The word "series" caught my attention. The capacitor should not be in
series with the crystal, it should be connected from the crystal pin to
ground. Having it in series would make the circuit very unstable.

There is also sometimes a second capacitor to ground on the second crystal
terminal. For low frequency crystals like this, there may also be a
resistor in series with the crystal. On timer chips designed for a 32khz
crystal, the resistor may be built into the chip so you don't need to add
it outside.
<tt>
resistor
xout -/\/\-o

---

;
| |
| === capacitor
32khz

---

|
xtal === Vss

---

|
|
xin

---

o
|
=== capacitor
|
Vss
</tt>

-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com