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Altera_Forum
Honored Contributor I
3,173 Views

assign two clocks

Hi, 

I have a design with two clocks. 

The first one (named "clk") is correctly assigned in my sdc file with: 

create_clock -name "clk" -period 5.000ns  

but my second clock (named "clock2") is generating a warning. I've tried to add: 

create_clock -name "clock2" -period 40.000ns  

but I still have a warning. "clock2" is not a port, it's generated from "clk". How should I assign that? 

Thank you.
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13 Replies
Altera_Forum
Honored Contributor I
267 Views

You need to use 'create_generated_clock'. 

 

See quartus help on create_generated_clock (http://quartushelp.altera.com/15.0/mergedprojects/tafs/tafs/tcl_pkg_sdc_ver_1.5_cmd_create_generated...). 

 

Cheers, 

Alex
Altera_Forum
Honored Contributor I
267 Views

create_generated_clock -source [get_ports {clk}] <relationship to source> <target> 

 

What structure in your design is creating clock2? If it's a PLL, you can use derive_pll_clocks to automatically create the generated clock. If not, you have to use create_generated_clock.
Altera_Forum
Honored Contributor I
267 Views

Thank you both but I still have another warning: 

 

Warning (332060): Node: *|clock2 was determined to be a clock but was found without an associated clock assignment. Warning (332061): Virtual clock clock2 is never referenced in any input or output delay assignment.
Altera_Forum
Honored Contributor I
267 Views

Hi, 

 

 

--- Quote Start ---  

Warning (332060): Node: *|clock2 was determined to be a clock but was found without an associated clock assignment. 

--- Quote End ---  

 

Is related to the timing constrains not satisfied. 

 

 

--- Quote Start ---  

Warning (332061): Virtual clock clock2 is never referenced in any input or output delay assignment. 

--- Quote End ---  

 

Check that the virtual clock is not originally intended as non-virtual clock and specify the clock targets. 

 

 

Already addressed in previous thread. 

 

Best Regards, 

Anand Raj Shankar 

(This message was posted on behalf of Intel Corporation)
Altera_Forum
Honored Contributor I
267 Views

A virtual clock is created if you use create_clock and then don't set a target (an input clock port). It's used with set_input_delay or set_output_delay as the reference clock that drives the "upstream" or "downstream" device to/from the FPGA. 

 

Post your updated sdc code.
Altera_Forum
Honored Contributor I
267 Views

Thank you both. 

Please tell me exactly what I have to do. 

Here is my sdc code: 

#************************************************************ # THIS IS A WIZARD-GENERATED FILE. # # Version 13.1.0 Build 162 10/23/2013 SJ Full Version # # ************************************************************ # Copyright (C) 1991-2013 Altera Corporation # Your use of Altera Corporation's design tools, logic functions # and other software and tools, and its AMPP partner logic # functions, and any output files from any of the foregoing # (including device programming or simulation files), and any # associated documentation or information are expressly subject # to the terms and conditions of the Altera Program License # Subscription Agreement, Altera MegaCore Function License # Agreement, or other applicable license agreement, including, # without limitation, that your use is for the sole purpose of # programming logic devices manufactured by Altera and sold by # Altera or its authorized distributors. Please refer to the # applicable agreement for further details. # Clock constraints create_clock -name "clk" -period 5.000ns create_generated_clock -divide_by 8 -name clock2 -source # Automatically constrain PLL and other generated clocks derive_pll_clocks -create_base_clocks # Automatically calculate clock uncertainty to jitter and other effects. derive_clock_uncertainty # tsu/th constraints # tco constraints # tpd constraints
Altera_Forum
Honored Contributor I
267 Views

The two warnings (note they are not errors) are directly related. You're creating a virtual clock but then not referencing it with any input or output port delay setting - set_input_delay or set_output_delay as sstrell suggested. 

 

Please note: it does not mean the timing constraint has not been satisfied, as suggested elsewhere in the thread. 

 

Post your updated constraints. 

 

Cheers, 

Alex
Altera_Forum
Honored Contributor I
267 Views

My sdc constraints are waiting for a moderator to agree...

Altera_Forum
Honored Contributor I
267 Views

 

--- Quote Start ---  

create_generated_clock -source [get_ports {clk}] <relationship to source> <target> 

 

What structure in your design is creating clock2? If it's a PLL, you can use derive_pll_clocks to automatically create the generated clock. If not, you have to use create_generated_clock. 

--- Quote End ---  

 

 

You need to answer this question before people can really help you. PLL? Did you try "derive_pll_clocks"? If the answer is yes to both these questions then you don't need the sdc constraints t for clock 2 at all (it is generated by "derive_pll_clocks")  

 

Obviously you don't have a 200MHz and 25MHz clock coming directly from the same clock pin?
Altera_Forum
Honored Contributor I
267 Views

You have not specified a target for your create_generated_clock constraint. That's why it's coming up as a virtual clock. If you are using a PLL, the target should be the output clock pin (get_pins) of the PLL, or just use derive_pll_clocks as mentioned.

Altera_Forum
Honored Contributor I
267 Views

I don't use a PLL or input/output pins as it's a simple internal clock divider. I really don't know how to resolve these warnings.

Altera_Forum
Honored Contributor I
267 Views

 

--- Quote Start ---  

I don't use a PLL or input/output pins as it's a simple internal clock divider. I really don't know how to resolve these warnings. 

--- Quote End ---  

 

 

The simplest and least problem clock divider is a PLL. A logic based clock divider can cause lots of timing problems as you can get a lot of skew on it. A PLL is almost always the simplest and best solution (not least as the constraints come for free with the derrive_pll_clocks SDC command)
Altera_Forum
Honored Contributor I
267 Views

OK, I'll try that but I have not the time today.

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