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steprog
12-23-2009, 05:45 PM
Hi Guys,
I got a project where Im synchronizing two props over a few feet distance. Im generating 60MHz + signals for modulating and the same for demodulation a few feet away. I was thinking of just using props for these clocks, but I need these to be sync up. Any suggestions?
Thanks, Greg

Ale
12-23-2009, 11:44 PM
One way could be to use the same clock for both. You may need to buffer it from one prop to the other one... I mean a 80 MHz clock not the 5 MHz one.

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Mike Green
12-23-2009, 11:49 PM
You might consider sending the clock and the datastream from the generating Prop to the receiving Prop. They'll go through similar drivers and similar cabling and the delays and signal degradation will be similar. You're probably going to run into trouble if you plan to just use the Prop's outputs rather than use a driver and receiver along with a matching cable.

You might also consider using some kind of self-clocking encoding of the signal (like Manchester encoding) rather than a separate clock and data.

steprog
12-24-2009, 01:17 AM
Why not just send the 5MHz to the XI pin would that work?

Harley
12-24-2009, 03:35 AM
steprog,

I solved this dilemna by using a 5 MHz crystal on one Prop, then generating a 5 MHz on one pin using a counter mode, and clocking the 2nd Prop on the X1 pin using FDS to communicate between the two Props. Saves a crystal, takes very little code to generate the 5 MHz output. http://forums.parallax.com/images/smilies/yeah.gif

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Harley Shanko

steprog
12-24-2009, 04:02 AM
I was thinking doing something like this, except I don't understand the FDS that you mentioned?

Toby Seckshund
12-24-2009, 04:36 AM
I tried to run two props off of one Xtal and it failed miserably. On reset of the "master" prop it disrupted the osc and crashed the secondary one. Presumably the osc stops whilst the prop goes through its RC Slow and RC Fast phases. ( works ok on AVRs )

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steprog
12-24-2009, 04:44 AM
Toby Seckshund said...
I tried to run two props off of one Xtal and it failed miserably. On reset of the "master" prop it disrupted the osc and crashed the secondary one. Presumably the osc stops whilst the prop goes through its RC Slow and RC Fast phases. ( works ok on AVRs )


I can understand that. The crystal drivers can be pretty sensitive to the load given.

Harley
12-24-2009, 04:49 AM
steprog said...
I was thinking doing something like this, except I don't understand the FDS that you mentioned?


FDS is Full Duplex Serial. I didn't have the spare I/Os to communicate in parallel between the two Props. FDS only required 2 I/Os each.

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Harley Shanko

Toby Seckshund
12-24-2009, 05:03 AM
@Steprog

I don't think it was a loading problem, the track was only 2cm long and when things were running all was well. When the secondary prop was reset, all was well. But when the primary prop was reset the secondary was left clockless for a bit and crashed. I just stiched another rock onto it, and all behaved themselves. As Ale says an external Xtal osc block would have worked ok. I didn't need syncronization, I was just trying to be sneeky/cheep.

I have done these things before on AVRs but of course they do not change their clock modes on reset.

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