Generating Video Signal w/ >5MHz Crystal

darco

I'm working on a ybox inspired board which uses a ENC28J60 instead of a (much more expensive) XPort for the networking magic. Space is a bit tight on the board for using thru-hole components, so I'm looking to save space on the board in whatever ways I can get away with.

One thing that would simplify the layout a bit would be to take advantage of the CLKOUT pin on the ENC28J60, and use that to drive the XI pin on the propeller. The frequency of the CLKOUT pin can be set to 3.125MHz, 6.25MHz, 8.333MHz, 12.5MHz, or 25MHz.

So my question is, how easily will I be able to output color NTSC video at these input clock rates?

Also, anyone have suggestions on what other things should be on a 'better' ybox-like board? Remember, this has to fit in an altoids tin...

Harrison.

It may be easier if you run the propeller on a 5mhz crystal so you won't run into any color ntsc issues. Then you can use one of the onboard propeller counters to synthesize a 25mhz clock for the enc28j60 chip. I did this when I didn't have a crystal for the enc28j60 and it worked fine.

Harrison

Mike Green

Although the Propeller is not guaranteed to work at over 80 MHz, it has been sold with a 6 MHz crystal and a clock frequency of 96 MHz with a PLLx16. With a 6.25 MHz input clock, this would give a clock frequency of 100 MHz which is likely (not certain) to work. Worse comes to worst, you can use the 3.125 MHz input clock for a clock frequency of 50 MHz (PLLx8) which certainly will work.

You should be able to generate fine NTSC baseband video with either clock frequency. Don't expect to be able to put out broadcast video at carrier frequencies that are not multiples of the input clock. There's a lot of jitter and other artifacts.

parts-man73

Darco,

I looked at your PCB design in the other thread you started. You have no crystal for the ENC28J60. Which is no problem, you can create a 25mhz clock with the Propeller, been done before and works fine that way, I assumed that is what you are doing.

But now you ask if you can eliminate the Propeller crystal and use the ENC28J60 to generate the clock for the Propeller? Where is the ENC28J60 going to get it's clock from? One chip or the other needs a Crystal, you can't eliminate both.

If it was me, I'd use a crystal on the Propeller, and feed the ENC28J60 a clock from the Propeller.

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Brian

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