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How to constraint min delay between LCELLs?

NuvKFC
New Contributor I
3,087 Views

Some asynchronous need to use LCELLs to generate pulses, like following example.

However, the delay of LCELLs is different on the difference FPGA device.

So, I use following constraints to constraint the min delay of LCELL. 

But it doesn't work.

How to constraint min delay between LCELLs?

Thank you very much.

 

Ex.:

lcell dly1d (.out(y1), .in(A) );

lcell dly4d (.out(y), .in(y1) );

 

Constraints:

set_min_delay 5.0 -from [get_pins dly1d|combout] \

                                  -to      [get_pins dly4d|combout]

set_net_delay -min 5.0  -from [get_pins dly1d|combout] \

                                            -to      [get_pins dly4d|combout]

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1 Solution
Nurina
Employee
2,723 Views

Hi,


Right so I don't think this constraint works on LCELLS alone. Because you would need a setup/hold relationship in the first place.

Which brings us back to using set_data_delay, which definitely should work on LCELL alone.

Since it isn't supported on Quartus II 13.1, I suggest you migrate to Quartus Standard 21.1 as this would have the set_data_delay.


On the other hand, you are using LCELL to generate a pulse to wake up the clocks right? Maybe you could consider using clock control IP: https://www.intel.com/content/dam/support/jp/ja/programmable/support-resources/bulk-container/pdfs/literature/ug/ug-altclkctrl.pdf


Regards,

Nurina


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26 Replies
sstrell
Honored Contributor III
2,449 Views

For minimum delay between, you would have your -from point to the output of one and your -to point to the input of the next.  You're pointing both to the outputs of both.

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NuvKFC
New Contributor I
2,430 Views

Hi sstrell
    Thank you, sstrell, very much.

How to know the LCELL's input after quartus synthesis for pointing the input to -to?.

 

Thank you very much.

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Nurina
Employee
2,440 Views

Hi, does the above reply help?


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NuvKFC
New Contributor I
2,430 Views

Hi Nurina
    Thank you, Nurina, very much.

How to know the LCELL's input after quartus synthesis for pointing the input to -to?.

 

Thank you very much.

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sstrell
Honored Contributor III
2,414 Views

The easiest way is to use the Name Finder, accessed when you use one of the GUI dialog boxes to create a timing constraint.  You can access them if you have your .sdc file open in the Quartus text editor (Edit menu -> Insert constraint).

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NuvKFC
New Contributor I
2,404 Views

Hi sstrell
    Thank you, sstrell, very much.

The Name Finder find signals of LCELL dly1d as follow picture. Which one is the input of the LCELL dly1d?

NuvKFC_0-1651326376332.png

 

 

Thank you very much.

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sstrell
Honored Contributor III
2,384 Views

The data ports are the inputs.  You can also look in the post-fit Technology Map Viewer to see these in a schematic view.

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NuvKFC
New Contributor I
2,361 Views

Hi sstrell
    Thank you, sstrell, very much.

 

I change the constraints to which are as follows.

 

Constraints:

set_min_delay 0.8 -from [get_pins dly*d|combout] \

                                  -to      [get_pins dly*d|data*]

set_net_delay -min 0.8  -from [get_pins dly*d|combout] \

                                            -to      [get_pins dly*d|data*]

 

After Quartus compile done, I report the relative path. The report command is as follow.

 

Report command:

report path -from [get_pins dly1d|combout] -to [get_pins dly4d|combout]

 

I saw the report. The net delay result isn't extended.

Does there have any problem of constraint setting?

 

Thank you very much.

 

 

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NuvKFC
New Contributor I
2,332 Views

Does any one know the root cause? Please help me. Thank you very much.

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Nurina
Employee
2,317 Views

Hello,


Apologies for the late response, I was on vacation.

Can you share your design so that I can reproduce the problem from my machine?


Regards,

Nurina


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NuvKFC
New Contributor I
2,298 Views

Hi Nurina

    Thank you, Nurina, very much. You may enjoy your vacation first^0^.

 

My design is as follows. I use LCELL to generate a pulse in a module without a clock. 

The design will be implemented on the advance node FPGA device, and it can't be modified.

On the advance node FPGA device, the delay of LCELL becomes shorter.

If the slew rate of A is too late, it may cause the generated pulse to fail. 

So, I want to constraint the min between A and Y4. 

I try to constraint the min delay between LCELLs, but it fail.

 

Design:

lcell dly1( .out(Y1), .in(A) );

lcell dly2( .out(Y2), .in(Y1) );

lcell dly3( .out(Y3), .in(Y2) );

lcell dly4( .out(Y4), .in(Y3) );

 

wire pulse = A & ~Y4;

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Nurina
Employee
2,286 Views

Hi,


Is there any reason why you don't want to create a pulse using flip flops? I don't think it's a good idea to create a pulse using LCELL primitive, its delay depends on hardware variables, so race conditions may happen thus creating an unreliable circuit: https://www.intel.com/content/www/us/en/programmable/quartushelp/17.0/hdl/prim/prim_file_lcell.htm


Regards,

Nurina


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NuvKFC
New Contributor I
2,253 Views

Hi Nurina

    Thank you, Nurina, very much.

 

Using LCELL to create a pulse is an unreliable circuit.

But the original SoC design has a sleep mode that all clocks are closed.
During that mode, I can only use LCELL to create a pulse to wake up clocks.

 

So, I need to constraint the LCELL delay to make the pulse width more robust.

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Nurina
Employee
2,233 Views

May I know which version of Quartus you are using? and which device are you using?


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NuvKFC
New Contributor I
2,219 Views

Hi Nurina

    Thank you, Nurina, very much.

 

I use Quartus13.1, and the used devices are Stratix III, Stratix IV and Stratix V.

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Nurina
Employee
2,169 Views

Hi,


Could you try set_data_delay instead of set_net_delay? I believe this should take place as you are using LCELL.


Regards,

Nurina


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NuvKFC
New Contributor I
2,163 Views

Hi Nurina

    Thank you, Nurina, very much.

 

Unfortunately, Quartus13.1 seem that don't support set_data_delay command.

NuvKFC_0-1654063094672.png


Have any others command can instead of set_net_delay?

 

Thank you very much.

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Nurina
Employee
2,141 Views

Hi,


Sorry about that, set_data_delay is pretty new in TimeQuest. I think the best you can do is use set_min/max_delay if you want to use LCELL. These two constraints would override default setup and hold relationships so your specification should be able to take place.


You may find this forum thread useful: https://community.intel.com/t5/Programmable-Devices/set-max-delay-vs-set-net-delay/m-p/232670


Regards,

Nurina


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NuvKFC
New Contributor I
2,133 Views

Hi Nurina
    Thank you, Nurina, very much.

 

I had tried following constraint to limit the min delay between LCELLs to more than 0.8 ns.

 

But it fail.

After compiling done, I report the path delay in TimeQuest, which is from the LCELLs output to another LCELLs output.

The delay time still small than 0.8 ns.

 

Constraints:

set_min_delay 0.8 -from [get_pins dly*d|combout] \

                                  -to      [get_pins dly*d|data*]

 

Report command:

report path -from [get_pins dly*d|combout] -to [get_pins dly*d|combout]

 

Design:

lcell dly1d( .out(Y1), .in(A) );

lcell dly2d( .out(Y2), .in(Y1) );

lcell dly3d( .out(Y3), .in(Y2) );

lcell dly4d( .out(Y4), .in(Y3) );

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Nurina
Employee
2,724 Views

Hi,


Right so I don't think this constraint works on LCELLS alone. Because you would need a setup/hold relationship in the first place.

Which brings us back to using set_data_delay, which definitely should work on LCELL alone.

Since it isn't supported on Quartus II 13.1, I suggest you migrate to Quartus Standard 21.1 as this would have the set_data_delay.


On the other hand, you are using LCELL to generate a pulse to wake up the clocks right? Maybe you could consider using clock control IP: https://www.intel.com/content/dam/support/jp/ja/programmable/support-resources/bulk-container/pdfs/literature/ug/ug-altclkctrl.pdf


Regards,

Nurina


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