Alchitry Cu External Clock

Help Request
1

Hello all,

I’ve been playing around with the original Alchitry Cu board and I have a project in mind for generating an NTSC compatible color video signal. This requires the circuitry to run at a multiple of the NTSC colorburst frequency (~3.5795 MHz). I’m targeting a frequency of 8 times that number, 28.636 MHz. I’ve run the icepll tool but am unable to find any values that will generate any precise multiple of the colorburst frequency with the PLL and onboard 100 MHz clock.

Can I just connect an external 28.636 MHz oscillator to one of the Cu’s IO pins and use it as the FPGA clock source? Are there any special considerations I would need to take with this approach, or is there a better way to do what I’m trying to do?

2

You can hook up an external clock. For something like this, that’s probably your best bet. The clock signal should be a 3.3V square wave and not just a crystal.

See this doc. You will want to use one of the pins that has the GCLK label on it. These are clock capable inputs that allow the signal to be routed efficiently though the whole FPGA.

3

Thanks for the response. I’ll try the external clock approach. It looks like the document you’ve linked requires approved access. Can you approve my access request when you get the chance?

4

My bad. I made it viewable by anyone.