RC oscillator and Quartz frequencies

[Emam]

Dear all,
I have a microcontroller for which I use a quartz crystal, and an RC oscillator circuit, both to produce an axact time measurement.
My quartz has a frequncy of around 32 KHZ and my RC oscillator around 33 KHZ.
I have a question regarding to the choice of my RC oscillator:

- before I had: R=200Kohm, C=300 pF
- Now I have R=100Kohm, C=150 pF

Now I see sometimes, I do not obtain an output for my microcontroller !
Does that change in RC circuit could be the reason?
And in general what is important in choosing RC components?

Many thanks

 

[Harald Kapp]

This will depend on the µC you are using. The datasheet gives the information which values for R and C are recommended. Without knwoing which µC you are using, we cannot tell whether your values for R and C are correct.

Note that an RC oscillator never will be as stable and exact as a quartz oscillator.

 

[Emam]

Thank you for your response. In fact in my data sheet, the only info I have is that the frequency of my RC oscillator should be in the window defined by the frequency of my quartz +/- 25%.
When I do the calculation I am inside this window.
No maybe sometimes I am not in this window, if the RC is not stable ? is it possible?
But what could be the reasons of the instability of RC circuit?
Thank you very much

 

[Harald Kapp]

How should we know? Tell us which microcontroller you are using, that's a good starting point for an analysis.

 

[LvW]

How should we know? Tell us which microcontroller you are using, that's a good starting point for an analysis.

And tell us which type of RC oscillator you are using.

 

[Harald Kapp]

And tell us which type of RC oscillator you are using.

It is probably one integrated into the µC. This is rather common today.

 

[Emam]

Sorry but its not a microcontroller. Its a kind of circuit designed for Specific purposes (ASIC).
That is why I need to put an RC circuit. and I put it because the quartz put some times to start. In this way I can measure times even before the quartz starts to run.
But The only thing I know is that my ASIC requests that the RC frequency is in the window defined by my quartz to do not loose any thing.
Now I dont know why the frequency of my RC shoud not be stable...and I dont know because of what? and I dont know how it can affect the circuit?

Many thanks and bests regards

 

[Harald Kapp]

In that case LvW's question is a good one: which kind of RC circuit are you using?

You are aware that an RC oscillator will take some time to startup, too, are you?

 

[davenn]

Now I dont know why the frequency of my RC shoud not be stable...and I dont know because of what? and I dont know how it can affect the circuit?

Because that's the nature of RC and LC oscillators. There is nothing to "fix" the frequency to a single frequency. They will drift up and down with changes in temperature, changing proximity to other things around them. They will be very susceptible to vibration ( this is called microphonics)

Often RC or LC oscillators will be covered in wax to help stabilise temperature and microphonics

This is why for all critical oscillators, the oscillator will be based around a quartz crystal ( which even then may be ovenised for temperature stability)

Dave