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Explorer
Explorer
4,672 Views
Registered: ‎05-03-2012

BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Hi ALL

 

How DISCRETE JITTER is calculated for output clock if I0 and I1 both connected to BUFGCTRL?

 

I get 335ns DJ while source clock has 80ps P-P JITTER.

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Explorer
Explorer
6,330 Views
Registered: ‎05-03-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Actually my answer after playing with 14.2 is:

par always take worse JITTER from I0 and I1 of BUFGMUX for output (no role for PRIORITY)

And now I agree with it

 

Thank you,

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6 Replies
Moderator
Moderator
4,670 Views
Registered: ‎01-16-2013

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Explorer
Explorer
4,668 Views
Registered: ‎05-03-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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yashp,

 

I think you didnt feel the quiestion.

 

This answer records say nothing about BUFGCTRL which gets two clocks as inputs.

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Xilinx Employee
Xilinx Employee
4,655 Views
Registered: ‎07-31-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Hi,

 

Give the priority keyword, based on, which clock you want to analyze the timing for. Check the example PERIOD constraint with the PRIORITY keyword below. Use PRIORITY on the faster clock. 

 

However generally i have used PRIORITY on BUFGMUX where it works. Havent't tested it on BUFGCTRL.

 

NET "clk1" TNM_NET = clk1;

TIMESPEC TS_clk_1 = PERIOD "clk1" 8 ns HIGH 50% PRIORITY 1;
NET "clk2" TNM_NET = clk2;
TIMESPEC TS_clk_2 = PERIOD "clk2" 10 ns HIGH 50% PRIORITY 2;
 
If you do not give any PRIORITY, ideally, the tool does a worst case analysis and calculates the timing based on the worst case parameters.
 
If the analysis is made on the worst case CLOCK, i.e the clock with minimum period, then you need not worry about the slow clock. 
 
Hope this makes it clear.
Thanks,
Anirudh

PS: Please MARK this as an answer in case it helped resolve your query.Give kudos in case the post guided you to a solution.
Xilinx Employee
Xilinx Employee
4,613 Views
Registered: ‎07-23-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Yash & Anirudh have answered your question to most extent.

When you have two clocks driving a BUFGCTRL, then based on the priority level defined on the clocks in the timing constraints, one of the two clocks will be taken into account for calculating the jitter or for timing analysis.

But when no period constraint is defined, the tool will pick the latest constraint in pcf i.e. if the period constraint of clk1 is defined before the period constraint of clk2 then clk2 will be considered for timing analysis.
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Explorer
Explorer
4,604 Views
Registered: ‎05-03-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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The question was - JITTER after BUFGCTRL.

I tried PRIORITY but always get

 

  Clock Uncertainty:    0.244ns

  Clock Uncertainty:          0.244ns  ((TSJ^2 + DJ^2)^1/2) / 2 + PE
    Total System Jitter (TSJ):  0.000ns
    Discrete Jitter (DJ):       0.335ns
    Phase Error (PE):           0.076ns

 

after BUFGMUX

 

and

 

  Clock Uncertainty:    0.117ns

  Clock Uncertainty:          0.117ns  ((TSJ^2 + DJ^2)^1/2) / 2 + PE
    Total System Jitter (TSJ):  0.000ns
    Discrete Jitter (DJ):       0.079ns
    Phase Error (PE):           0.076ns

 

without BUFGMUX.

 

and cant do anything with this.

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Explorer
Explorer
6,331 Views
Registered: ‎05-03-2012

Re: BUFGCTRL RESULT JITTER (DISCRETE JITTER)

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Actually my answer after playing with 14.2 is:

par always take worse JITTER from I0 and I1 of BUFGMUX for output (no role for PRIORITY)

And now I agree with it

 

Thank you,

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