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Beginner
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A code reviewer has suggested that my main 50MHz input clock from an external crystal oscillator should be input to a PLL within the FPGA to produce a cleaner internal 50MHz clock, i.e. glitches on the external clock will be ignored. Is this true?

I have only ever used a PLL within an FPGA to multiply, or divide, the incoming clock frequency, or phase shift the incoming clock. I have never used a PLL if the design only has one clock frequency.

A reviewer of my code has suggested that it is good practice to clock the internal logic from a PLL, not the external crystal oscillator clock input. Is there advantage to doing this?

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New Contributor I
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Hi,

 

In the FPGA, the clock must be connected to the dedicated clock input pins [clk0p/n, clk1p/n..... ] to get the external clock to the clock network of FPGA. But usage of PLL is not necessary unless you want to have some kind of clock synthesis.

However, the input clock connected to dedicated pins can be used in the design without any PLL.

 

module test (   input wire clock, input wire reset, output reg toggle );   always @ ( posedge clock) begin if (reset) begin toggle <= 1'b0; end else begin toggle <= ~ toggle; end end   endmodule

 

 

With Regards,

HPB

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Employee
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Hi Fergus Robertson

 

PLL provides synchronization between external clock and internal clocks. A slight change at external clock does not vary the PLL frequency.

 

When a PLL locks onto the input frequency, there is a limited variation of that signal at the output. This is good for jitter reduction and clock skew control of the design.

 

Thanks.

Eng Wei

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Employee
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Hi Fergus Robertson

 

Let me know if you have any further request on this topic before we go ahead to close the ticket.

 

Thanks.

Eng Wei

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Employee
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Hi Fergus Robertson

We do not receive any response from you to the previous feedback that I have provided. This thread will be transitioned to community support. If you have a new question, feel free to open a new thread to get the support from Intel experts. Otherwise, the community users will continue to help you on this thread. Thank you

 

Eng Wei.

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