Choosing a crystal

bsa22 · ‎2004-08-05

Posted on August 06, 2004 at 08:43

Choosing a crystal

bsa22 · ‎2004-08-02

Posted on August 03, 2004 at 02:26

I'm beginning to get together a parts list to order from Digikey for making a prototype using an ST7262. One thing that has me a little baffled is what crystal to choose. There are crystals, oscillators, ceramic crystals, crystal filters, etc. etc. And those are just the overall types!

I guess I've figured out that I want a regular old crystal, 12Mhz, through hole, HC49/US, or HC49/UA package.

Page 20 of the ST7262 datasheet advises on the selection and configuration

of the crystal. It gives recommended capacitance values based on the

''equivalent serial resistor.'' Citizen datasheets refer to ''equivalent serial

resistance'' which I assume is an equivalent term. It gives a value of 80

Ohms, while the datasheet gives capacitance values for 70 Ohms. The 80

value is a maximum for the range 10 to 14 Mhz, so I'm assuming this is

close enough to use the 70 Ohm column.

The citizen crystal has a load capacitance of 18pF, and there are also

crystals from ESR at 32pF and Series. What is the difference between

series and a number? Which do I want? Does this value affect the

recommendations given by the ST7262 data sheet?

Thanks!

pete3 · ‎2004-08-05

Posted on August 06, 2004 at 08:43

I suspect the series you refer to is the mode of the crystal. Some crystals prefer to be connected in either series or parallel resonant mode, which defines whether the capacitance you add should be in series or in paralled with the crystal. Depending on just how timing-critical your application is, this may not matter. The difference is usually negligible (

Be warned that if you choose the wrong value of capacitance you may cause undesirable side effects like resonating at 3x recommended frequency. This may also destroy your crystal under extreme circumstances.

You should be able to get away with using the 70 ohm column but know that there may be issues associated with it. If the value of capacitance is outside that recommended by the datasheet find another manufacturer.

http://www.circuitcellar.com/library/print/0298/bujanos91/8.htm gives a pretty comprehensive tutorial.

Oh, and good luck