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Role of capacitor in crystal circuit for micro-controller.

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sysysy

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Hi,

May i know what is the actual functional of capacitor that we use in the crystal circuit? for example 2 22pf are using in connect between 2 terminal of crystal to Gnd.

I remembered last time my friends take wrong value of the capacitor and connected to the crystal.As a result, the microcontroller is never getting work until he found out the problem.

From the datasheet, it juz say that for higher capacitance that we use, the re is more stability of oscillator but also increase the the start-up time. Unfortunately, it did not say why must use or real purpose of this capacitor.

Hope anyone know can share the discussion here.

Thanks


regards,

sysysy.
 

basically there are two reasons for using the capacitors with the crystal circuit

1. The oscillator consists of the inverter inside the PIC, the crystal, and external capacitance . The total phase shift around the loop (from one inverter terminal, through the inverter, across the crystal/capacitor network, back into the inverter) has to be either 0 or 360 (the same thing) degrees for oscillation. The capacitance adjusts the phase shift of the network to allow oscillation.

2. Crystals are designed to "see" a certain type of load, referred to as the Load Capacitance. In order for your crystal to operate at the correct frequency, it must see this value of capacitance at its terminals
 
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