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PIC 16F84A 32khz Crystal Capacitors 68 pF

 
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Regular Guy



Joined: 04 Jun 2017
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PIC 16F84A 32khz Crystal Capacitors 68 pF
PostPosted: Sat Jul 03, 2021 11:23 am     Reply with quote

Our PIC in Practice book calls for 68 pF capacitors with the 32khz crystal.

The Microchip capacitor guide only goes up to 33 pF for using with crystals.

What will happen when we stair step down with the capacitors from

68 to 56 then 47 then 39?

Thanks!

We are keeping the config word set a LP oscillator.
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newguy



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PostPosted: Sat Jul 03, 2021 2:57 pm     Reply with quote

Look for Ttelmah's post at the link below, 4th post in the thread: http://www.ccsinfo.com/forum/viewtopic.php?t=58595&highlight=load+crystal

Use the specification of your chosen crystal to figure out what the crystal caps should be.
Ttelmah



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PostPosted: Sun Jul 04, 2021 1:09 am     Reply with quote

The book will probably have a 'part number' for the crystal they intend to
be used. Now the commonest 32K crystals today are the so called 'watch'
crystals and are designed for low load capacitances like 6 to 15pF. These
are normally referred to as 'tuning fork cut types (XY). However there are
other crystal cuts available (though very uncommon), so you can get AT
and BT cut 32KHz crystals, and these commonly need much higher load
capacitances.
So it may be that the particular crystal the the book actually used, was a
type needing a much higher capacitance than normal.

Now the effect of the loading being 'wrong' will be first to cause inaccuracy
in the frequency, then at larger errors, it may even stop the crystal from
oscillating. As Newguy says, you need to start with the part number of the
crystal you actually 'have', Then make an estimate for the track
capacitances, add the pin capacitances (data sheet), and calculate the
correct value to use. The 'estimate' should be based on the width and
length of the tracks (formulae online for the capacitance of a track).
Regular Guy



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PostPosted: Sun Jul 04, 2021 9:48 am     Reply with quote

Thank you newguy and Ttelmah

Let me give our input.

1 The data sheets seem to sometimes have info on capacitors for crystals

and then other times have info for resistor and capacitors for the

RC circuits.

2 We did blowout one 32khz crystal.It may have been on a development

board. Most of our boards seem to have 22 pF capacitors soldered on.

3 We did get some hard to find 32khz 4 pin canned oscillators. We blew

one of those up too.

We are pretty set on doing the 'stair step' down experiment.

See how the circuit oscillates. See how long they last.

Thanks!
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Ttelmah



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PostPosted: Sun Jul 04, 2021 11:21 am     Reply with quote

It is very difficult to make a crystal fail in a circuit.
Mechanical shock (dropping it or hitting it).
Overheating when soldering.
Both much more likely to make a crystal fail.
Making it fail electrically would require applying quite high voltages.
Regular Guy



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PostPosted: Sun Jul 04, 2021 11:47 am     Reply with quote

Thank you Ttelmah

The 4 pin 32khz oscillator can may have been the one was on our

PICDEM 2 Plus.

We left the jumper for the RC 200khz circuit on.

When we had a 4 pin can in the socket.

They need to put a big red sticker on the board about that!

Noticed something on RC circuit was really hot.

By then it was too late.

The 32khz crystal was just on a breadboard.

Just stopped working.

Thanks!
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CCS 16F1459 PCWH Compiler

CCS 18F14K50 PCWH Compiler

ICD-U40

ICD-S40

Embedded Serial Busses Development Kit

dsPIC30F4012 Development Kit

16F877 Development Kit
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