ProcessorClockOscillatorCrystals

Including pin capacity in calculating crystal loading caps.

Paul B. Webster VK2BZC [paulb@MIDCOAST.COM.AU] says:

OK, the crystal has a rated load capacitance, which is what it wants to see for exact frequency. What it sees is this:
```Osc1 o------+--------+       Where:  C1 = loading cap on Osc1
|        |               C2 = pin capacity of Osc1 input
=== C1   === C2           C3 = loading cap on Osc2
|        |               C2 = pin capacity of Osc2 output
Crystal    +--------+-- Ground
|        |
=== C3   === C4
|        |
Osc2 o------+--------+
```

This is actually pretty simple because we make C1 = C3 and assume that C2 = C4 although for some peculiar reason, these are not specified in the datasheets. 10pF sounds a round guess however (implied from other CMOS spec sheets).

Let's say your crystal was specified at 15pF. This would be achieved with each of the pairs C1+C2 = C3+C4 = 30pf, i.e., two 30pF in parallel. If C2=C4 is approximately 10pF, then C1 and C3 would each be 20pF or in practice, a 22pF (nominal) capacitor.

If you really want accuracy, then C1 becomes a 8.2pF in series with a 3-30pF trimmer. Also, as the internal capacities C2 and C4 may vary with temperature and voltage, it is somewhat preferable if the loading capacitors are bigger rather than smaller. (This obviously influences crystal selection, but only for high accuracy.)

 file: /Techref/oscpinc.htm, 2KB, , updated: 2009/2/6 12:07, local time: 2023/9/27 09:17, TOP NEW HELP FIND:  3.235.188.113:LOG IN

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type a nice message (short messages are blocked as spam) in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.

Link? Put it here:
if you want a response, please enter your email address:
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
 Did you find what you needed? "No. I'm looking for: " "No. Take me to the search page." "No. Take me to the top so I can drill down by catagory" "No. I'm willing to pay for help, please refer me to a qualified consultant"

.