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Explorer
5,191 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.

1 Solution

Accepted Solutions
Explorer
6,849 Views
Registered: ‎05-03-2012

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,

6 Replies
Moderator
5,189 Views
Registered: ‎01-16-2013
Explorer
5,187 Views
Registered: ‎05-03-2012

yashp,

I think you didnt feel the quiestion.

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

Xilinx Employee
5,174 Views
Registered: ‎07-31-2012

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.
Community Manager
5,132 Views
Registered: ‎07-23-2012
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
5,123 Views
Registered: ‎05-03-2012

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.

Explorer
6,850 Views
Registered: ‎05-03-2012

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,