View Full Version : Hydra and 10MHz Crystal

09-17-2007, 08:59 PM
The Hydra uses a 10MHz crystal then code runs using PLL8x giving 80MHz execution speeds.

Can anyone tell me what the advantage is of doing that rather than using a 5MHz crystal and PLL16x ?

Mike Green
09-17-2007, 09:15 PM
There's no fundamental advantage in the case of the Hydra. For the SpinStamp, a 10MHz crystal is physically smaller than a 5MHz crystal and the decision to use a 10MHz crystal was based on physical mounting constraints.

09-17-2007, 09:19 PM
I have used both. I see no real difference. Isn't it that someone just happend to use 5 MHz and someone else 10 MHz?

On a very insignificant level, there is a slight difference in crystal drive current. Around 300 uA for 5 MHz and 500 uA for 10 MHz.


09-18-2007, 12:34 AM
I have used all kinds of crystals (and some oscillators as well); 10 MHz is actually the limit when using the PLL! But you can can use crystals with a much higher frequencies without the PLL.

Post Edited (deSilva) : 9/17/2007 7:52:55 PM GMT

09-18-2007, 02:40 AM
I Run crystal from old IBM PC moderboard.
And my Proto Boards (5) run fine.

| Proto) & pll8x))
| Proto) & 14_318_180)


09-18-2007, 02:54 AM
Yes, it will - but the PLL will no longer work http://forums.parallax.com/images/smilies/smile.gif Have a check: I am quite sure it will run with 14 MHz only, although you set 8x PLL.

However.... The PLLs of SOME Propellers might run with 12 MHz, perhaps some very few with 14.. ??

09-18-2007, 04:41 AM
Thanks, it's been one of those "why ?" mysteries which has been bugging me for a while.

@ Mike : I hadn't noticed the Spin Stamp also used 10MHz, thanks. Smaller footprint; that makes a lot of sense.