20 MHz + PLL vs 48MHz

[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

[mister_e]

By knowing what your project is... it will never be submit to extreme temperature variation... 20MHZ is more than enough safe. As now, mine works perfectly. I can't tell if it really worth to by a 48MHZ clock gen for a USB device. 20MHZ is safer than 4MHZ for sure... IMHO.

[Demon]

That's what I'm hoping.

This is designed to be used in PC gaming so it's meant for indoor use. I think the PC will act up before my circuit anyways.

This runs on 5V USB power at about 250 mA, I don't really have any heat issues on the circuit; LEDs, pushbuttons, 18F2550, 16F628, 24FC128, two MCP23016 and a graphic LCD.

Robert

[Darrel Taylor]

Honestly, I'd probably go with 20mhz too, but is there a difference between a 4mhz and 20mhz crystal?

Let's say you have a standard 50ppm 4mhz crystal.

It's minimum frequency would be 3,999,800 &nbsp; (4,000,000 - .005%)
The max would be 4,000,200 (4,000,000 + .005%)

Now we multiply that times 12 to get the 48mhz and the ...
Minimum = 47,997,600
Maximum = 48,002,400

Check the PPM...
Minimum = (1 - (48,000,000/47,997,600)) = -0.005%
Maximum = (1 - (48,000,000/48,002,400)) = 0.005%

They are still 50ppm difference.

So whether you have a 4mhz or 20mhz crystal, you still end up with the same 50ppm tolerance.
And with 4mhz you also get less emitted radiation, and other PCB problems.

Interesting eh?

DT

[mister_e]

All good points!
i agree Darrel but my biggest concern is more about the PLL reliability and stability.. i'm still afraid of it. Maybe i'm comming old