20 MHz + PLL vs 48MHz

[Demon]

What is the difference between using a 20 MHz crystal on a PIC 18F2550 running USB with this configuration:
CONFIG PLLDIV = 5 ;Divide by 5 (20MHz input)
CONFIG CPUDIV = OSC1_PLL2 ;[OSC1/OSC2 Src: /1][96MHz PLL Src: /2]
CONFIG USBDIV = 2 ;Clock source from 96MHz PLL/2
CONFIG FOSC = HSPLL_HS ;HS oscillator, PLL enabled, HS used by USB
DEFINE OSC 48 ' 20 MHz crystal with HSPLL enabled

and just using a 48 MHz crystal?

Is it more efficient? Is extra cost the only difference?

Also, what fuses would I need to change to use a 48 MHz crystal?

Thanks!

Robert

[Dave]

Demon, The reason for using a lower crystal frequency is to reduce the emitted rf from the oscillator circuit. The higher you go in main oscillator frequency the more possible harmonics you can produce thru the crystal circuit and the PCB traces.
Dave Purola,
N8NTA

[mister_e]

in another hand, lower your crystal is, more you need to multiply it's value with the PLL... wich in some case may be a killer. PLL may and will drift with temperature change...more you multiply more it will drift.

As now i didn't experiment too much problem with 20MHZ but i doubt one day i'll have. A real 48MHZ osc should be better but will probably never work good on a bread-board. Nothing is perfect.

[paul borgmeier]

What is the difference between using a 20 MHz crystal on a PIC 18F2550 running USB with this configuration: ... and just using a 48 MHz crystal?

from datasheet: HS Oscillator mode max = 25 MHz

Am I underdstanding what you are trying to do? If so, then 48 MHz Xtal is not a good option unless you like to overclock your devices.

[Darrel Taylor]

EC mode can take 48mhz input.

But it needs to be an external oscillator. Crystal won't work.
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[Demon]

Darrel,

I got some 48 MHz HC49 crystals from DigiKey along with the recommended caps. Why do you say crystals won't work and it must be an external oscillator? If it's a 48 MHz crystal, I don't undestand what might be the problem.

Or did you mean external RC won't work?

Robert

[Darrel Taylor]

Like Paul said. 25mhz is the max spec for the HS oscillator.

EC (External Clock) mode can take the 48mhz directly, but the chip can't generate it itself.

There are so many oscillator modes on the 2550, it's pretty amazing. You can even get 48mhz from a 4mhz crystal.
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[Demon]

Exactly, hence why I'm not sure which is the most efficient to use. Let me put it this way, if your life depended on it, which one would you chose?

- 20 MHz crystal boosted to 48 MHz using configuration settings.

or

- 48 MHz crystal.

This is not for a personal project, my goal is to one day market this creature and reliability is important to me.

Robert