frequency tolerance

[Hiroki]

There is no information about frequency tolerance in the Datasheet.

Would you provide the frequency tolerance of Vybrid?

Hiroki

[RossMcLuckie · Accepted solution]

By frequency tolerance I assume you are refering to the 24MHz and 32KHz input references?

Although these oscillators will support an input range, the system expects these values when generating all the associated clocks, so if you change the values you will alter the system operation, in some case make it non functional.

The crystal spec can be defined by PPM, based on the required clock interfaces, for example if using USB the 24 MHz clock should have a maximum PPM of +/-500.

You should verify the selected PPM meets all your intended interface clock specifications.

Ross

[Hiroki]

Ross,

Thank you for your reply.

Would you say that 24MHz and 32kHz oscillators can accept any PPM jitter crystal input however users need to consider system acceptable maximum PPM?

Hiroki

[RossMcLuckie]

Not sure I would say “any”, too open ended, better to state there are no special requirements and we support typical parts. Definitely the customer should consider the system wide PPM requirements, typical crystal PPM’s are in the range +/-50, other factors, such as layout will also contribute.

[ChrisNielsen]

I've always found it helpful to browse the app notes on the websites of crystal manufacturers -- lots of good information for begineers and experts.  Maybe search Digikey, Mouser, Newark to identify the companies supplying "in stock" parts, then check their websites.

For crystals, I've had success using this simple formula to set the 2 load cap values:

Cap = 2 * ( CLxtal - 3.5 stray )

CLxtal is the crystal datasheet load capacitance.

The 3.5 pF stray capacitance is ONLY typical for 2-4 layer 062 FR4 boards and it VARIES.  It is always best to design values on paper, build a few boards, measure, adjust.  Don't put a scope on the crystal pins or its load caps -- it will affect the measurement.  Have the CPU output the clock on another pin, hopefully with a large divisor so it's reasonably low in frequency, then put the scope there.

Chris