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Standard Delay Format (SDF) Timing Annotation
SDF for Mixed VHDL and Verilog Designs
The conditions are semantically equivalent and a match occurs. In contrast, path delay
conditions may be complicated and semantically equivalent conditions may not match. For
example,
Table 11-20. SDF Path Delay Conditions
SDF
Verilog
(COND (r1 || r2) (IOPATH clk q (5)))
if (r1 || r2) (clk => q) = 5; // matches
(COND (r1 || r2) (IOPATH clk q (5)))
if (r2 || r1) (clk => q) = 5; // does not match
The annotator does not match the second condition above because the order of r1 and r2 are
reversed.
Rounded Timing Values
The SDF TIMESCALE construct specifies time units of values in the SDF file. The annotator
rounds timing values from the SDF file to the time precision of the module that is annotated. For
example, if the SDF TIMESCALE is 1ns and a value of .016 is annotated to a path delay in a
module having a time precision of 10ps (from the timescale directive), then the path delay
receives a value of 20ps. The SDF value of 16ps is rounded to 20ps. Interconnect delays are
rounded to the time precision of the module that contains the annotated MIPD.
SDF for Mixed VHDL and Verilog Designs
Annotation of a mixed VHDL and Verilog design is very flexible. VHDL VITAL cells and
Verilog cells can be annotated from the same SDF file. This flexibility is available only by
using the simulator’s SDF command-line options. The Verilog $sdf_annotate system task can
annotate Verilog cells only. See the vsim command for more information on SDF commandline options.
Interconnect Delays
An interconnect delay represents the delay from the output of one device to the input of another.
ModelSim can model single interconnect delays or multisource interconnect delays for Verilog,
VHDL/VITAL, or mixed designs. See the vsim command for more information on the relevant
command-line arguments.
Timing checks are performed on the interconnect delayed versions of input ports. This may
result in misleading timing constraint violations, because the ports may satisfy the constraint
while the delayed versions may not. If the simulator seems to report incorrect violations, be sure
to account for the effect of interconnect delays.
ModelSim User’s Manual, v6.2g
February 2007
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