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How to change Au master clock speed through PLL - Printable Version

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How to change Au master clock speed through PLL - mdlougheed - 12-30-2020

Although it's alluded to, after searching the forum and internet too, I'm unable to find an example of changing the Au boards master clock speed through the on-board PLL.
  • 100MHz on-board clock (can be multiplied internally by the FPGA)
Can this be done using the ACF and XDC constraint files within Alchitry Labs (v1.2.5) or is there further setup within Vivado, in concert with AL that needs doing?  

For the projects I'm contemplating, it would be handy to multiply the clock by 150% and 200%.  For another it's desirable to multiply the clock by 2/3 - that is multiply by 2, then divided by 3.  The Xilinx App Note on Using Constraints hints at how to do this, however is not particularly straightforward, nor is it targeted at AL (which is understandable).

Any HOW-TO from the forum is appreciated.
MDL


RE: How to change Au master clock speed through PLL - alchitry - 01-14-2021

To do this, with an Au project open go to Project -> Vivado IP Catalog.

Wait a little for the catalog to open.

Navigate to FPGA Features and Design -> Clocking -> double click Clocking Wizard

You can now go through the wizard to setup the PLL.

Things to note, you can't use the main clock if you are also using the PLL. The routing in the FPGA can't route it to the general fabric and the PLL. To get around this, if you still need 100MHz, you can make the PLL output multiple frequencies including 100MHz.

The default clock input is 100MHz which is perfect.

You should only really need to mess with the "Output Clocks" tab and maybe "Port Renaming" if you want to change that. I usually name the clock outputs to clk_MHZ where MHZ is the frequency so like clk_200.

Once you're done with the wizard you'll be prompted to generate the output products. Click Generate.

On the bottom of the window you'll see it's progress. DON'T CLOSE THE WINDOW YET. You must wait for the synthesis to finish.

Once it shows 100% under "Progress" (shouldn't take very long) you can close the window.

Alchitry labs then scans for new cores. They should show up in the project tree. You should see a clk_wiz_0 core and a clk_wiz_0_stub.v file under it.

The stub file is the "black box" version of the core and is useful to look at to know what the ports are named.

If you didn't change much in the wizard, you should have a clock and reset input and a clock and locked output.

You may want to set the reset input connect to the reset button and use the locked output for the rest of your designs reset (inverted so it is reset when not locked). This will ensure a stable clock.

Use the clock output as you would any other clock.