20 MHz + PLL vs 48MHz

[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   (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

[Darrel Taylor]

It's a really interesting PLL on these chips.

The PLL's VCO always runs at 96mhz. And the crystals frequency is always divided down to 4mhz for the PLL reference. So in effect, the frequency of the crystal doesn't matter, only it's tolerance.

The only stability issue is the Jitter of +/- 0.25% But with either a 4mhz or 20mhz crystal, it's the same because it's inherent to the PLL.

The only way to remove the Jitter from the equation is to go back to the external clock (EC) and provide the 48mhz without using the PLL.

But, it won't really matter any which way you go. They'll all work just fine.

DT

[Demon]

... The PLL's VCO always runs at 96mhz. And the crystals frequency is always divided down to 4mhz for the PLL reference. So in effect, the frequency of the crystal doesn't matter, only it's tolerance....

DT

I noticed the division part, that makes a lot of sense. I just didn't know if there were other factors that would make a performance difference. So now I know that the tolerance is really what I'm concerned with when I'm selecting crystals.

I'll probably stick to 20 MHz components 'cause I've got lots in stock and I use them in other projects. But it is good to know that a 4 MHz crystal can be just as good for a 18F USB application.

Thanks guys!

Robert

[Dave]

Demon, Don't for get to view the power requirements for the the osc. circuit. The faster the crystal the more power it will draw. Although it sounds like you will be running it off the USB power, if it were to be battery powered this is something to consider.
Dave Purola,
N8NTA