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AR# 3224

FPGA Express 2.x/3.0: Comparators may not infer carry logic

Description

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

Urgency: Standard

General Description:
FPGA Express versions 2.x and 3.0 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.

This issue has been resolved with version 3.1 of FPGA Express.

Solution

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;
AR# 3224
Date 08/28/2001
Status Archive
Type General Article