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PICList Thread
'Oscillators, resistors, and Capacitors'
1995\10\20@115828 by Steve Hurd (719) 599-1778

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Hi there,

I have a couple of questions for the list.

How do you know which mode to use for your oscillator? (XT/HS)
In the data handbook it shows 4Mhz as both XT mode and HS mode.
I am currently using 10Mhz so I assume that HS is the mode I should use.

Also in the data book it mentions that a series resistor (RS) might be
required... It doesn't list a suggested value for RS and I am a bit unsure if I
need to use it.  I am using the ECS-100-s-4 crystal that is listed as one of
the test crystals in the data book.  I requested the data sheet for that
crystal from Digikey and the data sheet I got was fairly useless.

Still another question I have is the use of Cap's in the PIC circuits.  I know
there should be a cap near Vdd and currently I have about a 10mF planned.  How
about on the I/O lines?  Does there need to be Cap's on the I/O lines and if so
what size would be approriate so that it wouldn't slow down the I/O lines to
much.  My application will be driving an LCD Module from RS-232 info so I'm a
little concerned that if I put too large of a cap on the I/O lines that the
Serial line communication stuff may get messed up.

Thanks for the help
Steve
--
spam_OUThurds-cos1TakeThisOuTspamkaman.com

1995\10\21@032505 by Marc Laclair

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Marc Laclair                            |      A closed mind gathers no      |
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On Fri, 20 Oct 1995, Steve Hurd (719) 599-1778 wrote:

> Hi there,
>
> I have a couple of questions for the list.
>
> How do you know which mode to use for your oscillator? (XT/HS)
> In the data handbook it shows 4Mhz as both XT mode and HS mode.
> I am currently using 10Mhz so I assume that HS is the mode I should use.

correct HS is the proper mode.

>
> Also in the data book it mentions that a series resistor (RS) might be
> required... It doesn't list a suggested value for RS and I am a bit unsure if
  I
> need to use it.  I am using the ECS-100-s-4 crystal that is listed as one of
> the test crystals in the data book.  I requested the data sheet for that
> crystal from Digikey and the data sheet I got was fairly useless.

About a year and a half ago I had problems gitting a PIC to oscillate.
So, I call Microchip and tried to find out more what size R to use in series.
Microchip said ask the Crystal MFG. and guess what the Crystal MFG.
said?  Thats right, ask the Microcontroller MFG.  Talk about passing the
buck.  Any way, I can't remember how I came up with the value but the
current value we use in all our products is 80.6 ohms.  Some of the PICs
will oscillate with out it and some will not.

BTW: we use the epson 4MHz and 10Mhz crystals

{Quote hidden}

A .1mF is suffient for Vdd to Vss.  Your power supply my need a diff.
size cap.  As for the I/O lines for the RS-232 you can drive right into
232 driver chip or see the APP NOTE for the RS232 direct drive in the
Microchip Embbeded controler hand book.  You will not need caps on the
LCD lines either.


> Thanks for the help
> Steve
> --
> hurds-cos1spamKILLspamkaman.com
>


Cheers,

Marc La Clair

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