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GD76's avatar
GD76
Icon for Occasional Contributor rankOccasional Contributor
4 years ago
Solved

Looking for help on constraint syntax

There is a setup slack violation (see attached). The timing violation at RAM_CONTROL:i13|q3 to DSACK1. We are off by 2.477ns. Looking to understand what syntax for CPLD can be constructed to constrai...
  • sstrell's avatar
    sstrell
    4 years ago

    If I'm reading your document correctly, you're transferring between a 40 MHz clock domain and a 20 MHz clock domain. As such, you need a multicycle timing exception in your .sdc file. Going from a faster to a slower domain usually requires a -start multicycle to select the correct launch edge for the setup and hold analysis:

    set_multicycle_path –from SYSCLK –to CLK20MHZSYS_ext –setup –start 2

    set_multicycle_path –from SYSCLK –to CLK20MHZSYS_ext –hold –start 1

sstrell's avatar
sstrell
Icon for Super Contributor rankSuper Contributor
4 years ago

So you're launching on the falling edge and you want to latch not on the next rising edge but the rising edge after what's in your screenshot? So a period and a half? Maybe:

set_multicycle_path –fall_from SYSCLK –to CLK20MHZSYS_ext –setup –end 2

set_multicycle_path –fall_from SYSCLK –to CLK20MHZSYS_ext –hold –end 1

  • GD76's avatar
    GD76
    Icon for Occasional Contributor rankOccasional Contributor
    4 years ago

    DSACK1 is launching at falling edge, but want delay it to next falling edge. In this case half-period is 25ns, so 50ns.

    So, I have external clock as 50ns, half-period 25ns

    create_clock -name {SYSCLK} -period 50.000 -waveform { 0.000 25.00 } [get_ports {SYSCLK}]

    From the above, creating a virtual clock

    create_clock -name {CLK20MHZSYS_ext} -period 50.000

    The signal of interest constraint as below and give setup slack timing violation. How to make this delayed?

    set_output_delay -clock CLK20MHZSYS_ext -max 7.5 [get_ports {DSACK1_n}]
    set_output_delay -clock CLK20MHZSYS_ext -min 0.5 [get_ports {DSACK1_n}]

    Then tried add this but still give violation

    set_output_delay -clock CLK20MHZSYS_ext -clock_fall 50.000 [get_ports {DSACK1_n}]

    • sstrell's avatar
      sstrell
      Icon for Super Contributor rankSuper Contributor
      4 years ago

      -clock_fall is a switch indicating, for output delay, that data is captured on the falling edge of the virtual clock. I'm not sure why you have "50.000" there. You would still need -max and -min with 7.5 and 0.5, respectively.

      So I'm still not clear what you want. Are you saying you want this signal launched by a falling edge (SYSCLK) and latched by the next falling edge of CLK20MHZSYS_ext? If you clarify exactly what edges you want to use for launch and latch, maybe even a diagram, that would help.