Crystal Selection

[JimWalker]

Can anyone tell me why some crystals work and not others. I have a PIC16F877 and a PIC16F873 and both behave the same when using three crystals I have. Both PICs are set to use HS oscillator, and have 22pf capacitors to ground off each crystal pin.

Each crystal is your box standard shape, but the text on them is as follows:

Crystal 1 - This one works on both pics.
XT05360
4.000MHz
ECM 11E

Crystal 2 - This doesn't work on either - ie the PIC appears to not be receiving a clock signal.
AQ
10.000MHz

Crystal 3 - This doesn't work on either as the above.
10.0000
IQD CK
d8


Thanks in advance.

Jim

[kersing]

Hi,

Quoting from the PIC16F87X data sheet: "The PIC16F87X oscillator design requires the use of a parallel cut crystal."
So the non working crystals might be of a wrong type.

For a nice for a bit of background reading (quoting from the 18F14K50 data sheet):
• AN826, “Crystal Oscillator Basics and Crystal Selection for rfPIC® and PIC® Devices” (DS00826)
• AN849, “Basic PIC® Oscillator Design” (DS00849)
• AN943, “Practical PIC® Oscillator Analysis and Design” (DS00943)
• AN949, “Making Your Oscillator Work” (DS00949)

Regards,

Jac

[Benj]

Hello

There are figures assigned to crystals such as load capacatance and shunt capacitance. If these are too high then they can cause the device to fail. I've only ever had problems using the smaller surface mount types of crystals combined with the ARM processors as the tolerances on these seem to be much worse.

These crystals or similar should work fine with PIC, AVR and ARM processors.
http://www.rapidonline.com/Electronic-C ... ACT)/29219

I have however come across damaged or faulty crystals so this could be why you have one that wont work with anything.