prop as XTAL

Bobb Fwed

I know there has been talk about it before. But it is difficult to find things with the forum search engine.

I'm wondering what is need to run a Prop's xtal from another Prop. I'm not sure what the XTAL I/O pins are actually doing. I'm hoping it is as easy as wiring up a couple resistors between prop A output to prop B's xtals. Any cautions for running more than a couple Props off a single output?

Cluso99

You just connect the output pin to the XI pin(s) on the other props. Leave the XO unconnected. Be careful to run grounds beside the crystal line and prefeably run a lower speed (e.g. 5.00MHz) and use the internal pllx16 in the props. No resistors required. Probably you could happily drive 5 props, depending on total track length. Maybe Beau may have better info on this.

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Bobb Fwed

My idea is simply to have one prop running off a crystal to drive other props so they are all in sync. There is no need for protection resistors, one less part for me to mess with. Kind of the super computer idea that others have been bringing up.
Any more tips would be helpful.

Phil Pilgrim (PhiPi)

As long as you drive the slave XTI pins (leaving XTO open) from a port pin on the master Prop, you should be fine. Using the master Prop's XTO pin as the drive source would be a mistake, though, since it's not designed to handle a load bigger than a resonator of some sort. Also, keep your clock traces as short as possible.

If I were doing this, I would probably loop the clock line through the various XTIs, rather than using a branching topology, and put pads for a termination network near the end of the loop. Here is a paper that discusses clock line termination.

-Phil

mctrivia

to keep all in perfect sinc though would you not want all traces to reach there corresponding props at the same time. or at 80Mhz would propagation delay not be an issue.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Phil Pilgrim (PhiPi)

mctrivia,

The wavelength (in a vacuum) of an 80 MHz clock is 3.75 meters. So synchronicity on a small board should be less of a concern than reflections and overshoots. I have a feeling that the PLLs will add more phase uncertainty than the onboard propagation time does, but I could be wrong about that.

-Phil

mctrivia

true. I was thinking about the DVI connectors I have worked with where all wire lengths are the same to the mm. but that is significantly higher frequency.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.