Answers Database

M1 VERILOG/VHDL: CLB Flip-flops and latches may have zero setup delay inan SDF netlist for a routed design


Record #2558

Product Family:  Software

Product Line:  Merged Core

Problem Title:
M1 VERILOG/VHDL: CLB Flip-flops and latches may have zero setup delay
inan SDF netlist for a routed design



Problem Description:
Keywords: zero setup hold SDF Verilog VHDL flip-flop register latch, ngd2ver, ng
d2vhdl

Urgency: standard

General Description:
It is possible to find flip-flops or latches appearing
to have zero setup times in an SDF delay file.


Solution 1:

Zero setup delay may be commonly seen in the SDF file SETUP
records for flip-flops and latches in M1.

Example:

    (TIMINGCHECK
      (WIDTH (posedge CLK)  (2000:2000:2000))
      (WIDTH (negedge CLK)  (2000:2000:2000))
      (SETUP IN (posedge CLK) (0:0:0))
			       ^^^^^^^

The reason these zero SETUP delays occur is because the delay distribution algor
ithm in the M1 back-annotation tool
annotated 0 setup/hold times on flip-flops and latches and
annotates nonzero delay values to gates and carry logic
instead.  This should not be a source of concern because the
timing, when viewed at the external pins of the CLB (the only
place our speed files guarantee it), is correct.

On the other hand, CLB
flip-flops or latches with NO setup records in the SDF will
usually correspond to registers that have been disabled (i.e.
registers whose output is always a constant value due to their
clocks being disabled and their INIT values matching either
VCC or GND on their data inputs, etc.).  Such registers will
still appear in the netlist, but will have no timing
attributes whatsoever.

Additionally,

- It is also possible for flip-flops to have setup
requirements from the data input with respect to the clock,
but not have setup requirements from the CE or SET/RST inputs
to the clock if the CE and local SET/RST functions are not
used on a given instance of the flip-flop.

- If the local SR signal is used, it will have a setup
(recovery) constraint.

- A global reset (e.g., GSR) will not have a setup requirement
or SDF setup record.





End of Record #2558