a7 external clock

[frank2020]

Would like to use an external 100 MHz clock for the A7. Assume that we would use a pair of Artix MRCC pins, e.g. PIO36/PIO40 or PIO18/PIO19. How do we handle existing on-board clocks?

[jpeyron]

Hi @frank2020,

Are you talking about the Artix-7 or are you talking about the Cmod A7? Also depending on what you are trying to do this might be a helpful forum thread.

cheers,

Jon

[frank2020]

Jon,

Thanks for your reply.

We're using the Cmod A7. Checked the board and see that the ASEM1-100.000MHZ-LC-T is not populated. We'd like to clock the A7 with 100MHz LVCMOS; but, still be able to talk to it via USB. What would be the best way to setup clock domains?

[Alex]

It looks like external clock is not recommended for Cmod A7. 

[zygot]

@frank2020,

I may not be understanding your question but since you reference the unpopulated ASEM1-100.000MHZ-LC-T I'm assuming that you don't need your 100 MHz clock to be synchronous with an external component. Is there a reason why you don't want to use an MMCM to create a derived clock from the USB 12 MHz clock input? That's what I did in my CmodA735T demo. You can find it in the project vault. If you want to feed a clock into your design through one of the IO pins PIO36 on the Cmod A7-35T goes to a clock capable pin. Any MRCC, SRCC type _P IO pin will do.

CORRECTION!!! The following comment should be ignored...

The schematics aren't complete but it looks like you don't want to try soldering a ASEM1-100.000MHZ-LC-T component onto your board as its output is tied to the USB 12 MHz clock output through a 50 ohm resistor.

 

[frank2020]

Hello zygot,

Thanks for your reply. Actually we do want to have the 100 MHz clock synchronous with an an external 100 MHz reference; don't want to derive a 100 MHz clock from the 12 MHz USB input. We do want to be able to talk to the Cmod A7 via USB also.

Regards,

frank2020

[BYTEMAN]

On 18/6/2017 at 3:50 AM, zygot said:

@frank2020,

"---The schematics aren't complete but it looks like you don't want to try soldering a ASEM1-100.000MHZ-LC-T component onto your board as its output is tied to the USB 12 MHz clock output through a 50 ohm resistor."

But I think is possible to remove the R80 resistor in order to decuple the USB_12MHZ clock line from the GCLK and then soldering the U4 to use a 100 MHz clock source, I'm wrong?

Thank

Best regards

Grinch

[xc6lx45]

9 minutes ago, BYTEMAN said:

But I think is possible to remove the R80 resistor in order to decuple the USB_12MHZ clock line from the GCLK and then soldering the U4 to use a 100 MHz clock source, I'm wrong?

Yes, please check this post:

 

[zygot]

To all,

I got this one wrong. Unfortunately, I dealt with my faux pas on a different thread.... but @xc6lx45 is correct... as obviously others have found out. Unfortunately, when there are multiple threads talking about the same topic it's hard to backtrack and fix mistakes. Apologies to all.