Crystal Speeds to Drive Propellers..

KPR

I have a question about different Clock crystals..

So far in the forums I've seen people using the default 5.000 mhz ones and 6.000 and now 6.250 etc.. I know that the hydra uses a 10.000 mhz crystal as does the new web pror being developed..

So what is the best to use??

If I use a 10 mhz instead of a 5 do I change the multiplier, and what will a 10 give me over 5??

I can see that a 6.25 is 25% faster and that makes sense to me especially using it with a en28j60 to get the 25 mhz ..

Could someone enlighten me on the where's and why's to use each??

Thanks

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Leon

10 MHz (x8) has the advantage that the crystals are smaller. It's actually out of spec. for the Propeller oscillator (8 MHz max.) but they seem to work.

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Leon Heller
Amateur radio callsign: G1HSM

MacGeek117

6.25MHz @ 16x PLL gives you a 100MHz system clock.
10MHz is the highest recommended crystal speed, as far as I know.
MacGeek117

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hover1

The 6.25 MHz is nice because with a PLL of 16 gives a nice even 100 MHz. The Prop was only spec'd to do 80 MHz, but it was found to be happy at 100. That's why Parallax now sells the 6.25 crystals.

If you use a 10 MHz, PLL would be set at 8, (for 80 MHz)


Jim

Toby Seckshund

The default clock out of the en28j60 is 25MHz / 4 ie 6.25MHz so you can either clock the Prop from the en28j60 and get 100MHz, or generate the 25MHz from the Prop. Either way there is only one Xtal required

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KPR

Thanks for the explanation guys.. I can see why 6.25 mHz is the way to go for most applications, but this being said, why does the hydra and the upcoming web board use 10 mhz then?

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New ICON coming, gotta wait for the INK to heal, now we have colour!

Bill Henning

physically smaller, both those boards are smt

KPR said...
Thanks for the explanation guys.. I can see why 6.25 mHz is the way to go for most applications, but this being said, why does the hydra and the upcoming web board use 10 mhz then?

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Leon

See my post.

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Leon Heller
Amateur radio callsign: G1HSM

Cluso99

I use 6.5MHz for 104MHz for two reasons...
1. Overclocking
2. 6.5MHz xtal is easy to get from DigiKey

I have also used 6MHz. 6.25MHz is ideal if you want to calculate timing because the clock is 10ns.

All these overclocking ( >80MHz ) requires careful pcb design.

I also have 13.5MHz with PLL*8 for 108MHz overclocking (also easy from Digikey) but full testing has not been completed.

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