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Adventurer
Adventurer
9,271 Views
Registered: ‎06-05-2014

wrong txoutclk frequency for my gtx transceiver

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I have an ip core i've generated using the 7 series transceiver wizard. When I go through the wizard I specify 125 MHz as my reference clock frequency. However, when I run the core in hardware I see 156.25 MHz at the txoutclk. I've verified that my input clock is 125 MHz. I have this going through a buffer and then to gtrefclk0_in.

 

Why is the ip core I've generated changing the txout clk to 156.25 when it should be 125 MHz?

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Scholar dwisehart
Scholar
17,736 Views
Registered: ‎06-23-2013

Re: wrong txoutclk frequency for my gtx transceiver

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Yes, that is correct.

5 Replies
Scholar dwisehart
Scholar
9,267 Views
Registered: ‎06-23-2013

Re: wrong txoutclk frequency for my gtx transceiver

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What is the source of your TXOUTCLK?  See page 150 of UG476: there are several sources and different dividers available for TXOUTCLK.

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Adventurer
Adventurer
9,239 Views
Registered: ‎06-05-2014

Re: wrong txoutclk frequency for my gtx transceiver

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hmm, not sure I have control over that. When I use the wizard I spec a frequency of 125 MHz and a line rate of 3.125 GHz and it generates a bunch of files. The wrapper I've written to instantiate the <name>_GT module doesn't have any controllable parameters for the pll.

 

Is the tx_clk output always fixed at 156.25 MHz for some reason?

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Scholar dwisehart
Scholar
9,233 Views
Registered: ‎06-23-2013

Re: wrong txoutclk frequency for my gtx transceiver

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No, it is not always that frequency, but depending on the external and internal data widths along with the line rate, it might be a preferable frequency.

 

Look at the "Encoding and Clocking" tab of the GT Wizard.

 

Adventurer
Adventurer
9,230 Views
Registered: ‎06-05-2014

Re: wrong txoutclk frequency for my gtx transceiver

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oh, I think I get it now. Looking at the 7 series gtx transceiver guide Equation 3-1 the txusrclk rate is just the line rate over the internal data width. So, with a line rate of 3.125 GHz and a data width of 20 I should get 156.25 MHz, which I see on my scope. So, the txusrclk just depends on those two numbers and not on the input frequency. The wizard sets up pll based on input clk, line rate and data width. Does that all sound correct?

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Scholar dwisehart
Scholar
17,737 Views
Registered: ‎06-23-2013

Re: wrong txoutclk frequency for my gtx transceiver

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Yes, that is correct.