Answers Database

F1.4: FPGA Express 1.2/2.0: Comparators may not infer carry logic


Record #3224

Product Family:  Software

Product Line:  Synopsys

Problem Title:
F1.4: FPGA Express 1.2/2.0: Comparators may not infer carry logic


Problem Description:
Keywords: Express, compare, comparator, Verilog, VHDL, carry logic

Urgency: Standard

General Description:
FPGA Express versions 1.2 and 2.0 implement large comparators using general
combinatorial logic only by default, instead of using carry logic optimized
for Xilinx devices along with the required combinatorial logic.

Straight combinatorial logic may take up less space (fewer LookUp Tables), but
will run slower and have less predictable timing than an implementation using
carry logic.


Solution 1:

To force FPGA Express infers carry logic, use the following syntax when
describing compare functionality.

Verilog:

(A-B) >> 0;	    instead of	    A >> B;

VHDL:

(A-B) > 0;	    instead of	    A > B;



End of Record #3224