Capacitor Values for XTALS

[PJALM]

is it ok to use a 18pf Capacitor for any crystals? I have hundreds and wanted to use some up.

[PJALM]

Wow, I thought this would be an easy question to get an answer for but I guess I was wrong :(

[HenrikOlsson]

You need to look at the datasheet for the particular crystal you're aiming to use in order to find the recomended load capacitance. A Digikey search on crystals with 18pF load capacitance brings up ~4000 items so you should be able to find one that you can use with your capacitors.

If you already have the crystal (and no datasheet for it) then why not simply try it. It will most likely work but the frequency might be a bit off depending on how far from it's ideal load capacitance value you are - try it.

[Darrel Taylor]

Microchip always specifies the capacitor values as (reference only).

Final capacitor selection requires a bit of testing as outlined in this Application Note ...

AN949 - Making Your Oscillator Work
http://ww1.microchip.com/downloads/en/AppNotes/00949a.pdf

Here's the first two paragraphs ...

INTRODUCTION

Many Microchip customers ask us for help when
designing their oscillator circuits. Due to the great number
of combinations of VDD and temperature ranges,
crystal or resonator, loading capacitors, circuit layout
and circuit board materials, it is not practical to provide
a cookbook process that would work well for everyone
or even for most users. However, there are measures
that can be used to develop and test for good working
oscillator circuits.

This application note breaks down the oscillator design
process into the following three parts: specifying the
operating conditions, selecting the components and
testing oscillator performance. Other concerns are also
discussed that may be helpful during layout and
testing.

[PJALM]

Would it be better to stick with oscillators? I have even more of those, got 500 of the 48MHZ oscilators from ebay a few years ago for $5, been using them every since but they take a lot of PCB space.

[cncmachineguy]

Color me silly, but I have always just used the internal osc. Never had any issues yet. And for $5, if you use 1 or 2 of them you could feel good about the purchase. then build some tree decorations with the rest. That way the $5 purchase won't continue to cost in design, board space, pins, and headache.

just my $.02

I am sure I will invoke lots of replys citing the accuracy of the internal osc. Bring them on. IMHO there are but a handful of applacitations that require greater than the internal will provide.

[PJALM]

I would love to get away with just using internal OSC but most of my applications make use of the USB functions so it won't work :(

I am not too worried about using up pins, i have gotten pretty good at multiplexing I/Os, its amazing what can be done with a simple 8 pin PIC if you take your time and multiplex it to death lol :)

To be honest I have not really used crystals or resonators all that much, the company I work for always insisted on using oscillators for their accuracy not to mention they have internal parts to adjust for temperature variances etc (they also only use 1 pin). Only reason I was asking is because I am leaving that company soon and will continue making projects for myself so I will no longer have the same funding and I am told oscillators are quite costly. Then only crystals I managed to save was some SMD 20MHz and 4MHz (ruffly 500 of each) and the only caps i found in my home stock was these SMD 805 Series 18pF ones. Kinda hard to just try em out to see if they work but I will try to sneak a test board into production before I leave the company, hopefully they won't notice (bad me lol).

Anyways, thanks for the great advise everyone. :D

[mackrackit]

Kinda hard to just try em out to see if they work

Get a Sharpie :)
http://www.picbasic.co.uk/forum/content.php?r=105-SMD-Prototyping

[PJALM]

Cool I will have to try that one day, don't have any etching materials right now but I did ask my boss and he said if there is any extra space on any boards I am making then I am more them welcome to use it to make whatever I want so I am doing that today. Better to just let them make it for me since we have the machines in shop to do it including solder masks. :D