proper 32kHz crystal

Forum 2005-2010 (read only) Hardware Development
1

I intend to run an arduino with a 32kHz crystal connected. That is the processor will run with the internal oscillator and thus I can use a 32kHz crystal. The datasheet and application note AVR042 are not very specific about proper crystall specification.

They just say the oscillator is optimized for "standard" 32 kHz watch crystals and provides 20 pF for the internal capacitators. It does not tell though what a good choice for the crystals would be.

In my catalog I can order capacitive load values CL = 6, 7, 9, 12,5 pF and ESR = 20-70kOhm. I have no clue what a good choice would be.

My guess would be to buy a 7pF 60kOhm Pmax = 0.5uW. Basically because this is cheap, in stock and very tiny. However there are other cheap alternatives as well.

Has anyone any hint for me how to determine the proper "standard" parameters?

Thanks, Udo

2

I guess that you should say what you need the 32K crystal for.

If it is for a real time clock application ppm is the parameter to look for.
The lower the better (and probably the more expensive).

There IC RTCs available with I2C or SPI interface if you would prefer to use one of those.

cheers

3

The goal is to get power consumption as low a possible while retaining reasonable accurate timing. At least more accurate than the internal RC oscillator.

So 20ppm (which is what almost all specifiy) is fine with me. My concern is that the oscillator might not start under low voltage conditions (~1.8V). So I wonder if I should go for more or less capacitance.

Accurarcy is appreciated but not my main concern.

So should I opt more for 7, 9 or 12pF? Price is not the biggest issue either. The crystal will be the cheapest part of the whole thing (maybe except for the resistors),

Udo

4

Take a look at this:

and go for the lower load capacitance :slight_smile:

5

Today I am lucky. I found this one:

http://www.crystekcrystals.com/crystal/appnotes/PierceGateLoadCap.pdf.

and this:

http://www.foxonline.com/pdfs/xtaldesignnotes.pdf

So I infer I should go for 12.5pF. I will start with this and see how it turns out.

Udo

6

Nice references !

Thanks for posting :slight_smile:

7

Finally I found out what I really need. Of course Atmel refers to the datasheets but the datasheets are not very clear. The really useful information resides here:

AVR532: Migration from ATmega48/88/168 to ATmega48A/88A/168A

AVR505: Migration between ATmega16/32 and ATmega164P/324P/644P

AVR042: AVR Hardware Design Considerations

It just never occured to me that the migration notes could hold such useful information while the datasheet does not. Now it is trivial to select the proper crystal and the proper load balancers. Do not ask what I will use. If you need the information read these 3 notes they contain everything that is needed.

Udo