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PICList Thread
'crystal configs'
1996\05\13@045537 by n/a

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Hi,

  Does anyone have any info on the different oscillator options. Is the
particular option totally dependent upon the speed of the crystal that
you're using i.e. for a 2Mhz crystal use the XT option, as (sort of) defined
in Table 5.4.2 (Capacitor Selection Table) of the data sheet, or are there
other factors to consider. Could I use the HS option instead? Finally, here
are my guesses as to what the letters mean:

    RC - Resistor/Capacitor
    LP - Low Power (or is it Low Precision)
    XT - Extra something or other
    HS - High Speed (High Stability?)

I'd be grateful for any info on this,


             Geoff.

1996\05\13@053558 by fastfwd

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Geoff Wootton <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> wrote:

> Does anyone have any info on the different oscillator options. Is
> the particular option totally dependent upon the speed of the
> crystal that you're using i.e. for a 2Mhz crystal use the XT
> option, as (sort of) defined in Table 5.4.2 (Capacitor Selection
> Table) of the data sheet, or are there other factors to consider.
> Could I use the HS option instead? Finally, here are my guesses as
> to what the letters mean:
>
>      RC - Resistor/Capacitor
>      LP - Low Power (or is it Low Precision)
>      XT - Extra something or other
>      HS - High Speed (High Stability?)
>
> I'd be grateful for any info on this,

Geoff:

You were close:

   RC = Resistor/Capacitor
   LP = Low Power Crystal
   XT = Crystal
   HS = High Speed Crystal

Each oscillator-type is associated with a certain maximum speed:  RC
and XT have a maximum speed of 4 MHz, LP's maximum is 200 KHz, and
HT's is 20 MHz.

If you're using an external clock (not just a crystal with two caps,
but a circuit that generates a clock frequency all by itself), you
can program your PIC to any oscillator type whose maximum speed is
above the speed of your clock.

If, on the other hand, you're using a crystal or ceramic resonator
alone (and relying on the PIC to generate the clock), there are
MINIMUM speeds as well.  Here's how it all breaks down:

Osc Mode    External Clock        Internal Clock

  RC         0 -   4 MHz          0     -   4 MHz
  LP         0 - 200 KHz          5 KHz - 200 KHz
  XT         0 -   4 MHz        100 KHz -   4 MHz
  HS (-04)   0 -   4 MHz          4 MHz -   4 MHz
  HS (-10)   0 -  10 MHz          4 MHz -  10 MHz
  HS (-20)   0 -  20 MHz          4 MHz -  20 MHz

I'd choose an oscillator type by going through the following list of
questions, stopping as soon as I answered "Yes":

   R/C Oscillator?  RC and a "-04" PIC.
   External Clock at 4 MHz or slower?  XT and a "-04" PIC.
   Crystal < 200 KHz?  LP and a "-04" PIC.
   Crystal > 200 KHz and <= 4 MHz?  XT and a "-04" PIC.
   External Clock or Crystal > 4 MHz, <= 10 MHz?  HS and a "-10" PIC.
   External Clock or Crystal > 10 MHz?  HS and a "-20" PIC.

-Andy

Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

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