ModelSim SE User's Manual - Electrical and Computer Engineering

UM-136 5 - Verilog simulation
Negative timing check limits
ModelSim SE User’s Manual
Verilog supports negative limit values in the $setuphold and $recrem system tasks. These
tasks have optional delayed versions of input signals to insure proper evaluation of models
with negative timing check limits. Delay values for these delayed nets are determined by
the simulator so that valid data is available for evaluation before a clocking signal.
Example
$setuphold(posedge clk, negedge d, 5, -3, Notifier,,, clk_dly, d_dly);
d violation 5
region
clk
ModelSim calculates the delay for signal d_dly as 4 time units instead of 3. It does this to
prevent d_dly and clk_dly from occurring simultaneously when a violation isn’t reported.
ModelSim accepts negative limit checks by default, unlike current versions of Verilog-XL.
To match Verilog-XL default behavior (i.e., zeroing all negative timing check limits), use
the +no_neg_tcheck argument to vsim (CR-373).
Negative timing constraint algorithm
The algorithm ModelSim uses to calculate delays for delayed nets isn’t described in IEEE
Std 1364. Rather, ModelSim matches Verilog-XL behavior. The algorithm attempts to find
a set of delays so the data net is valid when the clock net transitions and the timing checks
are satisfied. The algorithm is iterative because a set of delays can be selected that satisfies
all timing checks for a pair of inputs but then causes mis-ordering of another pair (where
both pairs of inputs share a common input). When a set of delays that satisfies all timing
checks is found, the delays are said to converge.
Verilog-XL compatible simulator arguments
The simulator arguments listed below are equivalent to Verilog-XL arguments and may
ease the porting of a design to ModelSim. See the vsim command (CR-373) for a description
of each argument.
+alt_path_delays
-l
+maxdelays
+mindelays
+multisource_int_delays
+no_cancelled_e_msg
+no_neg_tchk
+no_notifier
+no_path_edge
+no_pulse_msg
-no_risefall_delaynets
+no_show_cancelled_e
+nosdfwarn
+nowarn
+ntc_warn
+pulse_e/
+pulse_e_style_ondetect
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