Hello, I have an ALTCLKCTRL component implemented in my design targeting a Cyclone V FPGA. The ALTCLKCTRL has 4 of its inputs occupied: 1. Clock_1 driven by an FPGA pin. 2. Clock_2 driven by an FPGA pin. 3. Clock_3 coming from a pll_a. 4. Clock_4 coming from a pll_b. All of the above are from the same bank and connected correctly - the design passes fitting without issue and last but not least - it works. However, I do have some trouble with timing analysis: I use the "create_clock" SDC command for creating Clock_1 and Clock_2 and then the "derive_pll_clocks" macro to generate Clock_3 , Clock_4 and the output of ALTCLKCTRL. The result: Clock_1, Clock_2, Clock_3, Clock_4 get recognized as clocks by the tool but the output of ALTCLKCTRL doesn't. From all I read the "derive_pll_clocks" macro should apply to ANY clock sourcing element...So why doesn't it work for ALTCLKCTRL ?

No. derive_pll_clocks, as the name implies, only works for PLL clock outputs. Is only 1 of the 4 clocks active in the design at a time? If so, you also need a set_clock_groups timing exception because only one of the 4 clocks will be active on the output of the mux at a time. It would look something like: set_clock_groups -exclusive -group clock_1 -group clock_2 -group clock_3 -group clock_4 If any of the clocks bypass ALTCLKCTRL and are used in the design when a clock from the mux is active, you may need to add additional generated clock commands targeted to the output of the mux and then add set_clock_groups exceptions between them. Steve

Suppose only one of the clocks is active at a given time. Why "set_clock_groups" is enough ? Shouldn't I also apply the "create_generated_clock" for the ALTCLKCTRL's output ? Surely the ALTCLKCTRL component has some delay from input to output.

It does have delay, but you don't need to define the delay since the tool knows about internal resource delays. Clock creation is for creating essentially reference points for timing analysis, not defining delays. You can define delays if you want, but it's not necessary for internal paths. Using set_clock_groups will tell TimeQuest that only one clock is active at a time, the way your design is supposed to work.

--- Quote Start --- Clock creation is for creating essentially reference points for timing analysis, not defining delays. --- Quote End --- Exactly. If I don't use the "create_generated_clock" command on the output of ALTCLKCTRL - how would the tool know that it should analyze timing in reference to that point ? I mean, we use the "create_generated_clock" on all PLL outputs - so why is the ALTCLKCTRL any different ? Why doesn't it deserve a "create_generated_clock" command on it's output ?

There's nothing stopping you from doing it. Are you saying that the tool is reporting that the output of ALTCLKCTRL is an unconstrained clock? Like I said, if you have other clocks that don't go through the mux, you would need to create generated clocks on the output of the clock mux as references. The reason you don't need to in your case is that ALTCLKCTRL is not generating a new clock. PLLS do create new clocks based off of other clocks. But if for some reason the tool is reporting it as unconstrained (which is unusual), then yes you'd need generated clocks as well.

SDC for ALTCLKCTRL | Altera Community

13 Replies

Altera_Forum
Honored Contributor
9 years ago
No. derive_pll_clocks, as the name implies, only works for PLL clock outputs. Is only 1 of the 4 clocks active in the design at a time? If so, you also need a set_clock_groups timing exception because only one of the 4 clocks will be active on the output of the mux at a time. It would look something like:

set_clock_groups -exclusive -group clock_1 -group clock_2 -group clock_3 -group clock_4

If any of the clocks bypass ALTCLKCTRL and are used in the design when a clock from the mux is active, you may need to add additional generated clock commands targeted to the output of the mux and then add set_clock_groups exceptions between them.

Steve
Altera_Forum
Honored Contributor
9 years ago
Suppose only one of the clocks is active at a given time.
Why "set_clock_groups" is enough ? Shouldn't I also apply the "create_generated_clock" for the ALTCLKCTRL's output ?

Surely the ALTCLKCTRL component has some delay from input to output.
Altera_Forum
Honored Contributor
9 years ago
It does have delay, but you don't need to define the delay since the tool knows about internal resource delays. Clock creation is for creating essentially reference points for timing analysis, not defining delays. You can define delays if you want, but it's not necessary for internal paths. Using set_clock_groups will tell TimeQuest that only one clock is active at a time, the way your design is supposed to work.
Altera_Forum
Honored Contributor
9 years ago
--- Quote Start ---
Clock creation is for creating essentially reference points for timing analysis, not defining delays.
--- Quote End ---

Exactly. If I don't use the "create_generated_clock" command on the output of ALTCLKCTRL - how would the tool know that it should analyze timing in reference to that point ?

I mean, we use the "create_generated_clock" on all PLL outputs - so why is the ALTCLKCTRL any different ? Why doesn't it deserve a "create_generated_clock" command on it's output ?
Altera_Forum
Honored Contributor
9 years ago
There's nothing stopping you from doing it. Are you saying that the tool is reporting that the output of ALTCLKCTRL is an unconstrained clock? Like I said, if you have other clocks that don't go through the mux, you would need to create generated clocks on the output of the clock mux as references. The reason you don't need to in your case is that ALTCLKCTRL is not generating a new clock. PLLS do create new clocks based off of other clocks.

But if for some reason the tool is reporting it as unconstrained (which is unusual), then yes you'd need generated clocks as well.
Altera_Forum
Honored Contributor
9 years ago
--- Quote Start ---
Are you saying that the tool is reporting that the output of ALTCLKCTRL is an unconstrained clock?.
--- Quote End ---

Not exactly and that's where it gets strange...
In the "Clock Status Summary" it doesn't show at all (not even as a red unconstrained path).
I don't see in the "Clock Tree" report either. All other clocks are there, the output of ALTCLKCTRL isn't...

But despite the phantom behavior the MUX and the design it drives work.
Altera_Forum
Honored Contributor
9 years ago
--- Quote Start ---
Not exactly and that's where it gets strange...
In the "Clock Status Summary" it doesn't show at all (not even as a red unconstrained path).
I don't see in the "Clock Tree" report either. All other clocks are there, the output of ALTCLKCTRL isn't...

But despite the phantom behavior the MUX and the design it drives work.
--- Quote End ---

you will find all details here:
https://www.mixdown.ca/redmine/attachments/download/71/timing_analysis_of_internally_generated_clocks_in_timequest_v2.0.pdf

figure 19 (not really figure but SDC
Altera_Forum
Honored Contributor
9 years ago
Thanks, but this doesn't explain why TimeQuest fails to identify the output of ALTCLKCTRL as a clock.
Altera_Forum
Honored Contributor
9 years ago
Excluding a bug - the only explanation I can think of is this:
For timing purposes, the tool doesn't treat the output of ALTCLKCTRL as a clock source. Instead, it views it ALTCLKCTRL as a balanced delay in the clock path for all MUX inputs.
I.E: nothing more than a "long wire" leading to the input of the global clock tree.
Therefore, it's never considered as an additional clock source.

Do you agree with this assumption ?
Altera_Forum
Honored Contributor
9 years ago
That sounds right. The key is what you see in the unconstrained paths report. If it's not there, it's not an issue. But you should still have set_clock_groups for the output of the mux.

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SDC for ALTCLKCTRL

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