Answers Database

XC3000/XC3000/XC4000/XC5200: Logic cells and the FPGA Density Cross Reference Guide


Record #1897

Problem Title:
XC3000/XC3000/XC4000/XC5200: Logic cells and the FPGA Density Cross
Reference Guide



Problem Description:
Keywords:  logic, cell, clb

Urgency:  standard

General Description:  What are Logic Cells?  How do they relate
to different device architectures including those of different
vendors?


Solution 1:

A "logic cell" (LC) is the primary building block of today's
high density FPGAs and is the best measure of a device's
maximum logic capacity.  Logic Cell utilization will vary by
design and product family, thus making the total number of
Logic Cells the best "first pass" capacity comparison metric.

A logic cell consists of a 4-input lookup table and a flip
flop.

If each 4-input LUT and each register are considered 1/2 of a
logic cell, and a 3-input LUT has 3/4 the functionality of a
4 input LUT then each 3 input LUT can be considered 3/8 of a
logic cell.  This each xc4000 has 2.375 logic cells.

For more information, as well a comparison of different vendors, and architectur
es refer to:
http://www.xilinx.com/xbrf/xbrf011.pdf



End of Record #1897