[SI-LIST] Re: Crystal osciiator not oscillating
- From: "PRAJIT S NAIR" <prajitsnair@xxxxxxxxx>
- To: manishsn@xxxxxx
- Date: Wed, 23 May 2007 11:50:22 +0900
Dear Shivdas
Crystal Oscillator basically consists of an amplifier and a feedback
network. Oscillation occurs for a particular frequency if the loop gain is
unity or more and the total phaseshift is zero or some multiple of 360
degree.
Generally it is classified as startup state and steady state condition. The
startup phase mainly depends on the small signal loop gain of the circuit
and the steady state depends on the large signal properties of the circuit.
In your case, it seems that you have problem in Startup phase. Some of the
factors to my knowledge which can cause it are:
- Â? Q factor of your crystal oscillator may be low and is influenced
by the external circuitry.
- Â? Sometimes onchip amplifier also dictates the value of external
capacitance. Check the datasheet of crystal oscillator.
- Â? did you check the load capacitance requirement ? What is the value
prescribed in Datasheet? Higher value means large startup time.
With Warm Wishes
Prajit
On 5/22/07, Narayane Manish Shivdas, TLS,Chennai <manishsn@xxxxxx> wrote:
>
>
> Hi Gurus,
>
>
>
> In newly designed board, I am facing a problem of crystal
> oscillator not oscillating. This crystal is placed in between the XTAL
> &EXTALpin of the microcontroller. Also the load capacitors of 15pF
> populated.
>
> uC has internal inverter. Input and output of the inverter has
> 1MegaOhm resistor &series resistance of 475E at the output of the
> inverter.
>
> After so many trials on the boards I found out that there are
> two
> cases in which crystal start oscillating –
>
>
>
> 1) Lowering the value of the external load capacitors form 15pF to
> 1pF.
>
> 2) By keeping the load capacitance value same as 15pF, a 100K
> externalresistor between the XTAL &EXTAL pin of the microcontroller,
> population starts oscillations.
>
>
>
> What will be the reason because of that crystal is not able to oscillate?
>
>
>
> Thanks &Regards,
>
> Manish Narayane
>
> HCL Tech, chennai, India
>
>
>
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