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How to constraint "Auto Global clock" to use closer global clocks?

New Contributor I

The clocks structure and constraint are as follows.

Clock_a --------> Clock_a1


                    -----> Clock_a2


creat_clock -name Clock_a [get_ports Clock_a ]

create_generated_clock -name Clock_a1

create_generated_clock -name Clock_a2


After Fitter in Quartus with "Auto Global clock", all clocks are on the global clock.

Unfortunately, TimeQuest report hold time violations by getting large clock skew because Quartus choose a far away  global clock location for those clocks.


How to fix it under "Auto Global clock" case?

Had someone had the experience to share?

Could someone can help me to fix it, please?

Thank you very much.

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4 Replies
Honored Contributor III

If Clock_a1 and Clock_a2 are identical to Clock_a (i.e. not going through a PLL or other clock control logic), you don't need those generated clock constraints or even those clocks at all.  By creating separate clock domains from Clock_a, the compiler is putting Clock_a on a global.  Then it takes a branch of Clock_a after it has come off the global to go back to another clock control block to create the other domains on separate global resources.  This is probably why you are seeing the issue.  Just use Clock_a for everything.

New Contributor I

Hi sstrell

    Thank you, sstrell, very much.

Unfortunately, the design I get which is designed by RD use digital divider to divide the Clock_a to the Clock_a1 and the Clock_a2.

(Designer don't use the coding style of the clock enable to design the design because the design style is ASIC.

(And Unfortunately, I can't modify the design.


So, Should I still don't use the "create_generated_clock" command to create Clock_a1 and Clock_a2?


If I don't create Clock_a1 and Clock_a2, Quartus will report some node that "no clocks feeds this register's clock port".


Honored Contributor III

I don't understand.  So are Clock_a1 and Clock_a2 identical to Clock_a or are they different?  You say "use digital divider to divide" so I don't know if you mean just splitting one source clock into two or if this is like a divide frequency by 2 or something.  What are the frequencies of all 3 clocks?

If you can't modify the design, I would just turn off Auto Global for Clock_a1 and Clock_a2.  You would still need the generated clock constraints, but they are not complete.  They should be like:

create_generated_clock -name Clock_a1 -source Clock_a -multiply_by 1 (or -divide_by if the frequency is divided)

create_generated_clock -name Clock_a2 -source Clock_a -multiply_by 1

New Contributor I

Hi sstrell

     Sorry that ambiguity. The clocks structure and constraint are as follows.

Clock_a -------------------------------------> Clock_a for IP1


                    ---clock divider (Clock_a / 2)--> Clock_a1 for IP2


                    ---clock divider (Clock_a / 4)--> Clock_a2  for IP3


creat_clock -name Clock_a [get_ports Clock_a ]

create_generated_clock -name Clock_a1  -source Clock_a -divide_by 2

create_generated_clock -name Clock_a2  -source Clock_a -divide_by 4


As you said, maybe turning off Auto Global is the best choice.


Thank you very, very much.