![]() ![]() 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. P := ((i / 16) * (i mod 16) - (i mod 2)) mod 64 Ĭonstant is_prime: lut6to1_t := is_prime_f For LUT based FPGAs the target development of novel systems for rapid system prototyping circuit element is the k-input LUT. The way that FPGAs are able to do Boolean algebra is by using Look-Up Tables (LUTs). 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 At first, the output values for each combination of input variables constituting the Boolean Function are stored in the SRAM cells of the LUT. But it is partly your fault: you described all this as plain arithmetic while you wanted a constant look-up table. FPGA makes use of its LUTs as a preliminary resource to implement any logical function. Vivado is probably really implementing some arithmetic from your first version. 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. If (d_in(5 downto 4) * d_in(3 downto 0) - d_in(0) = primes(i)) then Proposed pipelined multipliers use 4252 fewer LUTs. ![]() If the value for address 01 was a 1, then the LUT would still perform as expected but the equivalent logic circuit. If the delays of all the connections between the. ![]() used as combinational function generators (one LUT is marked F. Type t_int_array is array (natural range) of integer Ĭonstant primes : t_int_array(0 to 17) := (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61) The circuit is specific to modern Xilinx FPGAs that are based on a 6-input LUT architecture. It doesnt have to model a single logic gate. Previous work reports a 3 improvement of the average critical path delay of a subset of VTR circuits 18, 26. FPGA (Field Programmable Gate Array) is an integrated circuit containing a matrix of. 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. ![]()
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