Crystal frequency for PIC18F4550

russk2txb · Joined: 30 Mar 2008 Posts: 109 Location: New Jersey

I'm looking for some advice on selecting the best crystal frequency. The Microchip data sheets give various configurations but there seems to be no suggestions why one would be used compared to another. For example, it appears that I can use a 48 MHz crystal and no divide for the PLL or MCU clock (in HS mode). Would that be best? Or is it better to use the 20 MHz crystal and use PLL / 5 and CPU / 2 in HSPLL mode? I guess the question boils down to, is it better to use PLL and a low frequency crystal?

Thanks, Russ

Ttelmah · Joined: 11 Mar 2010 Posts: 19549

You can't use a 48MHz crystal. Highest frequency supported by the oscillator circuitry, is 25MHz. Above 25MHz, you have to use an external _oscillator_, not just a crystal.
For 'fast' USB, you can use any mutliple of 4MHz, up to 24MHz.
The USB PLL, seems _slightly_ more reliable, when fed with a divided source, rather than directly from a crystal. So, 8MHz, 12Mhz, 16MHz, 20MHz, and 24MHz.
Generally, lower frequency crystals, reduce the risk of RF noise problems, however if the board is well designed, this is a minor.
Realistically choosing between these possible crystal values comes down to price/availability, possible duplication of stock if used in other projects, reducing the number of component lines needed, etc. etc..

Best Wishes

russk2txb · Joined: 30 Mar 2008 Posts: 109 Location: New Jersey

Thanks Ttelmah. I was leaning towards the 20 MHz crystal anyway as that's what I used in another project. I did not see where it says that 25 MHz is the highest frequency crystal that can be used.

Thanks, again,
Russ

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

KnowMore · Joined: 05 Jul 2010 Posts: 9 Location: Vietnam

Ttelmah · Joined: 11 Mar 2010 Posts: 19549

Don't confuse the internal oscillator, with the clock input.
If you look first at the table of crystals/oscillator configurations (2-3), you will see that the top three lines, using /10, /12, and no division att all (direct feed), are only listed for EC, ECIO, ECPLL, and ECPIO modes. These modes use an external _oscillator_. All the ones using 'HS', or 'HSPLL' (or XT), and the internal _oscillator_ block, are below 24MHz.
If you look at the table Ckielstra points you to, you will see that the clock is specified to run at 48MHz, but _not_ using a crystal.
If you have used it at this, you have been lucky. It is common for the oscillator to work well past spec, _but_ should not be relied on....

Best Wishes

KnowMore · Joined: 05 Jul 2010 Posts: 9 Location: Vietnam