Different crystals

[Frank607]

While LIVE looking how they replace a necessary cooling module on the ISS ( really nice ! )
, I think about a question

It’s about crystals:

What is the difference between a crystal from 20 MHz and 19.6608 MHz ( yes, I know speed )

In Flowcode you have a standard option for 19.6608 MHz Why not 20 MHz ? What is the technical difference ?

[brandonb]

i dont know but i would imagine it would have to do with assemby code and instruction cycles, the 19.6604 crystals dont have a very good ppm value as they are about 30, i find the 16 mhz crystal has low ppm of 10 and the flowcode delays are very accurate at 16mhz, check the a 16.384 mhz crystal http://www.digikey.com/product-detail/e ... ND/2207665 , you get even numbers with timer interrupts and can hit numbers like 100,1000 directly, i did a internet search of why 19.6604 is the standard but came up with nothing
here is a long list of different values if curious
http://www.telluriantech.com/standard_frequencies.html

[Frank607]

Thanks Brandon.

[medelec35]

Hi frank,
its all to do with timer interrupt values.
If you set your clock speed to the standard 19660800 then drag an interrupt component macro on your flowchart.
Select Timer0 and click on properties.
Observe the interrupt frequency for all the Prescaler rates.

Change clock speed to 20000000 and again Observe the interrupt frequency for all the Prescaler rates.

That should answer the question.
Brandon is also correct about 16.384 MHz crystal except on the lowest Prescaler rates
19.6608 is on that page Brandon posted a link for.

So I guess Matrix used the highest crystal value that will produce accurate interrupt timer values, but not too high to cause stray capacitance issues?

Martin