Relation between LUTs, logic cell, logic elements, system gates
My question is related to difference and relationship between these terminology:
- LUTs
- Logic Cell
- Logic Elements(LE)
- System Gates
I know that a lot depends on FPGA producer, but for example an FPGA vendor that state of one of it's products this: FPGA with 100.000 System Gates equivalent aprox. to 1.5k LE.
How I can relate this statement with equivalent FPGA vendor that use LUTs and Logic Cells?
I ask this question becouse I want to migrate some project from an FPGA that tell me System Gates number and LE to an FPGA that tell me it's LUTs number.
Answer
LUT, Logic Cell and Logic Element are all the same to me: the most basic FPGA general logic primitive. Xilinx use LUT, Altera LE, microsemi/lattice possibly something else.
The problem is, they are not the same. In their most recent architecture, Xilinx use 6-input LUT and altera 4-input LUT. They are aggregated in logic blocks which has other features like fast-carry chain, registers and distributed memory.
Converting to system gates is useful, but don't forget it's also a marketing war. A Xilinx FPGA should fit 1.5 times the logic of an Altera FPGA, since it's LUT have 6 instead of 4, right? Well, it largely depends on the design, if the design can't use 6-inputs much, the unused ones are wasted. Same with fast-carry logic, I don't know if they count that in equivalent gate number, but be advised that number is inflated.
System gates is a common measure of ASIC design complexity. The same design on two different foundries should have similar system gates number, as waste is not really an issue for ASIC.
If you're looking for an FPGA. I suggest you choose your vendor, port enough of your design to get an idea of how big a FPGA you need and choose a FPGA with an upgrade path (if you want to market). If it's for a single prototype, just use the biggest FPGA you can afford.