"Logic Cell" is a marketing number which replaces the old "System Gates" and is intended
to show the logic density of the device as compared to another device with a 4-input LUT
plus Flip-Flop architecture. Spartan 6 has 6-input LUT's paired with two Flip-flops. Each
of these is counted as 1.6 "logic cells" because some marketing guru decided that you
can fit the equivalent logic of 1.6 4-input LUT's + 1.6 Flops in the Spartan 6 quarter-slice.
As you were no-doubt warned about using "system gates" when selecting a device,
your mileage may vary. It's clear that the 6-LUT + 2 flops should hold more logic
than a 4-LUT plus 1 flop, however the multiplier may be smaller than 1.6 if your logic is
not very complex (already at one logic level) or higher for more complex logic. The
only way to know if your design will fit in a given device is to run it through the tools.
I personally eschew the "Logic Cell" rating and look right at the LUT count when deciding
on a device.
-- Gabor
-- Gabor
