Extra clock apart from 32Mhz


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hello everyone.. I have a papillio pro spartan 6 board. In this board, 32Mhz clock at pin 'P94' is available... However, I need a extra clock for my application, so I connected 20 Mhz clock from function generator and connected to pin 'P123"... But it doesn't function..I mean is this pin can work as a clock?




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not every pin can function as clock input. See here



page 30, search for "GCLK" in the name.


Now did you know that there are PLLs inside the FPGA? It takes 2 min of work (maybe a bit more if you do it the first time) to run the core generator and turn the existing 32M clock into 20 MHz or whatever you like.

It's one of the most useful features, IMO, in everyday use.

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just to be clear, P123 "should" work, at least to my understanding. Otherwise you'd get a warning (not an error) and it would probably still do the job, at least at 20 MHz, when the tool routes the signal across the yard via the back door through the kitchen into a clock network.

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Offroad.. Thank you for replying... Yes.. I can do with PLL... Just curious about that.. If one needs to use external clock then how it can be used? I am not yet successful with P123 pin... I need to check warnings as I don't get any errors... Will try  "CLOCK_DEDICATED_ROUTE=FALSE;" also...


Thanks to hamster for replying...

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true, I forgot that... an error that can be turned into a warning. Anyway, the pin in question should work as regular clock pin in the first place, at least for the FPGA (didn't check where it's going on the board).

I'd have a look at the electrical settings (an accidental pullup resistor? Correct voltage threshold, i.e. 3.3 V?).

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Well, for an input there actually isn't much I could do wrong.

See here


page 24:

Most likely, "LVTTL" is the correct setting in the .ucf file for this input.


Check the input signal with an oscilloscope, whether the source is accidentally AC-coupled.

What I'd do is write a small counter and drive the pin as output, to double-check that connections didn't get mixed up.

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