FPGA LUT CIRCUIT MOD
FPGA LUT CIRCUIT CODE

Note that any other 64x1 constant, even arbitrarily complex to compute, would lead to the same resources usage after synthesis. A simple 6-to-1 LUT is thus sufficient instead of a uselessly complex arithmetic-based circuit. The main difference is that the synthesizer first computes the is_prime constant just like a simulator would do and then uses it to infers the hardware. For example, to create a 2-input C-element with inputs A and B and output C, it suffices to implement a OR-AND network (you can do this with an FPGA LUT4) where C is the output of a 3-input AND and each of the inputs of this AND connects to the output of a 2. The logical functionality is the easy part.

FPGA LUT CIRCUIT MOD

P := ((i / 16) * (i mod 16) - (i mod 2)) mod 64 Ĭonstant is_prime: lut6to1_t := is_prime_f Implementing a C-element on LUT-based FPGAs can be done, although it is not very simple. Variable l: lut6to1_t := (others => '0') Why not computing your constant look-up table and then use it as is? The following is just an untested draft to show you an example of this functionally equivalent solution but significantly different on a pure synthesis point of view: type lut6to1_t is array(0 to 63) of std_ulogic Proposed pipelined multipliers use 4252 fewer LUTs. But it is partly your fault: you described all this as plain arithmetic while you wanted a constant look-up table. The circuit is specific to modern Xilinx FPGAs that are based on a 6-input LUT architecture. Vivado is probably really implementing some arithmetic from your first version.

FPGA LUT CIRCUIT CODE

When I change the code to just skip one subtraction: if (d_in(5 downto 4) * d_in(3 downto 0) = primes(i)) thenĪnd synthesize, I get the expected 1 LUT utilized. The block in LUT-based logic is made up of a D flip flop, look-up table, and 2:1 multiplexer. If (d_in(5 downto 4) * d_in(3 downto 0) - d_in(0) = primes(i)) then In other words, if a circuit card or board is created and an FPGA is a component of the circuit, the FPGA is programmed during the creation process but can subsequently be reprogrammed to reflect any modifications. The way that FPGAs are able to do Boolean algebra is by using Look-Up Tables (LUTs).

When I synthesize the following code, however, I get a report saying 7 LUTs have been utilized. Presumably, any function I have with 6 bits input, 1 bit output I can implement by utilizing only one LUT. On Artix-7, one LUT is 6-bit input, 1-bit output.

Fpga lut circuit