Logic utilization reported by the Quartus® II software is an estimation of how full the device is. It is given as a percentage, calculated from the number of half-adaptive logic modules (half-ALMs) available in the device, and the number of half-ALMs used in your design. In the Fitter report, the terms Combinational ALUT/register pairs and Combinational ALUT/register/register triples are used to indicate half-ALMs. Each ALM in Stratix® IV, Arria® II, Cyclone® IV or earlier families has 2 combinational logic LUTs and 2 registers, and they are paired off as combinational ALUT/register pairs. In Stratix V, Arria V and Cyclone V devices, there are two combinational LUTs and 4 registers per ALM, and they are grouped as combinational ALUT/register/register triples.
Logic utilization is calculated by estimating how many half-ALMs are needed to fit a design, and expressing it as a percentage of the total number of half-ALMs available in the FPGA. Logic utilization is a good representation of how full a device is, rather than the register utilization percentage or the combinational logic utilization percentatge because it considers the aspects described below.
The logic utilization metric is calculated as
- ( A - B C ) / (Total number of half-ALMs in the device)
where A, B, and C are defined as follows:
- A: Combinational ALUT/register pairs used in final Placement or Combinational ALUT/register/register triples used in final Placement
This is the actual number of completely or partially used half-ALMs in the design after placement
- B: Estimated pairs recoverable by pairing ALUTs and registers as design grows or Estimated triples recoverable by pairing ALUTs and registers as design grows
This is the Fitter's estimate of how many half-ALMs could be freed up by taking half-ALMs that only use the combinational ALUT part, and half-ALMs that only use the registers, and grouping them up so that each half-ALM uses both the Combinational ALUT and register resources. For each possible grouping, a half-ALM is recovered. Therefore, this quantity is shown as a negative number in the report. Using this quantity helps to estimate how much logic would be needed if the Fitter could pack all logic as densely as possible. Note that this type of packing may not give the optimal result for routability and timing performance.
- C: Estimated Combinational ALUT/register pairs unavailable or Estimated Combinational ALUT/register/register tripes unavailable
Due to a variety if reasons, the Fitter can not perfectly pack all the logic into every device resource. For example some half-ALMs cannot be paired up with other half-ALMs due to the number of inputs used, and some LABs cannot be fully packed with 10 ALMs due to routing restrictions. This quantity is the best estimate of the physical resources that will become unavailable for use, in terms of half-ALMs.