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calculate the capacitance for 8051

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Newbie level 6
Feb 4, 2006
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how to capacitance using microcontroller

I am using 8051 for my project. In books it is given to use 33pico farads capacitors at the 11.0592 MHz crystal. My doubt is why only we use 33pf at the oscillator cant we use capacitors of other value ??? And if so how to find that 33 pf is suitable for a particular frequency??? any method to find how much capacitance is needed???

how calculate the capacitance by microcontroller

If the on-chip amplifier is a simple inverter, such as in the 8051, the user can select values for CX1 and CX2 between some 20 and 100 pF, depending on whether start-up time or frequency stability is the more critical parameter in a specific application.

Quoted from "Oscillators for Microcontrollers":
**broken link removed**
where you will find detailed analysis on how to select/calculate these values ..


calculate capacitance required for given load

Essentially, look for the load capacitance of the crystal, given in the datasheet. A very common value is 18pF and this is generally tru for a wide range of crystals and frequencies.

The two capacitors appear in series (one pin of each is grounded) across the crystal. Their capacitance, plus any stray capacitance should equal the load capacitance of the crystal.

Thus, 33pF/2=16.5pF from the two caps, to which you add a couple of pF for the stray capacitance and you get something very close to the required 18pF.
why 33pf 8051

These capacitors are used to ensure oscillation. The value is NOT critical. Any value from 15pF to 40pF can do (22pF to 30pF recomended). Sometime it works even without capacitors - the stay capacitance is large enough to start oscillation.
why oscillation does not start in 8051

thanks for all who shared their ideas.
I used 18 pf and in some boards it is working and in some its not so i used a less value of 10pf it is better but it is also giving the start up problem. im not able to dump my code. So I removed both the caps its working.
But I should use some caps as a filterbed.

8051 at cut

This does not sound right. The oscillator should have those capacitors.
I would check carefully the crystals, to make sure they are parallel resonant crystals and they have not been damaged (fractured). If they are AT-strip crystals, these can easily be overdriven. In that case, I would install 33pF caps, but from the output of the oscillator to the crystal I would connect a resistor. Typically, a few hundred ohms should suffice. (Actually the resistor's value should be equal to the impedance, in ohms, of one of the 33pF caps at the crystal frequency).

With HC-49 AT-cut crystals, you should have no problem running with 33pF.

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