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???
05/07/05 08:51
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Msg Score: +1
 +1 Informative
#93159 - Better use 22MHz fundamental + two 27pF
Responding to: ???'s previous message
From the datasheet, the crystal circuit capacitor is given as 30pf+/-10%. i put 33pf. but i am getting only 8MHz for 24MHz crystal between XTAL1 & GND. and between XTAL1 & XTAL2 i am getting only 50Hz.

Like others already mentioned, you probably have an overtone quartz. There are ways to make this quartz work, by adding this LC-trap for instance, but I wouldn't recommend to do this, because it makes the oscillator consuming more PCB area and by this producing more interference and ground noise. And if the L isn't properly choosen, then you will never have a reliable oscillator...
I would use a fundamental quartz at 22MHz in combination with two 27pF burden capacitors.

By the way, putting a scope directly to the XTAL pins isn't a good idea. Due to unavoidable load capacitance of probe you will never see the performance of original oscillator. Then even a perfectly working oscillator seems to work unproperly.
It's better to connect the input of a free inverter (which sits very close to oscillator!) to the XTAL pin of interest and to connect the scope to the output of this gate.
If you have a special FET-probe providing extremely low load capacitance (<5pF), then, of course, you can directly connect it to the XTAL pins. But have the ground wire of probe as short as ever possible, less than 1cm! Otherwise you get 'pigtail effects', where due to resonances formed by inductivity of probe wires the shape of oscillation will look totally strange...

Kai

List of 11 messages in thread
TopicAuthorDate
crystal problem - 89c51            01/01/70 00:00      
   ALE            01/01/70 00:00      
      Maybe...            01/01/70 00:00      
   two capacitors or one??            01/01/70 00:00      
      milli?            01/01/70 00:00      
   read Atmel            01/01/70 00:00      
   Use XTAL2 pin for freq. measurment            01/01/70 00:00      
   Better use 22MHz fundamental + two 27pF            01/01/70 00:00      
      tried for low capacitance            01/01/70 00:00      
         bible time            01/01/70 00:00      
   This should help            01/01/70 00:00      

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