Initialization and Assignment from Constants (Literals) - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

You can initialize ap_[u]fixed variable with normal floating point constants of the usual C/C++ width:

  • 32 bits for type float
  • 64 bits for type double

That is, typically, a floating point value that is single precision type or in the form of double precision.

Note that the value assigned to the fixed-point variable will be limited by the precision of the constant. Use string initialization as described in Initialization and Assignment from Constants (Literals) to ensure that all bits of the fixed-point variable are populated according to the precision described by the string.


#include <ap_fixed.h>

ap_ufixed<30, 15> my15BitInt = 3.1415;
ap_fixed<42, 23> my42BitInt = -1158.987;
ap_ufixed<99, 40> = 287432.0382911;
ap_fixed<36,30> = -0x123.456p-1;

The ap_[u]fixed types do not support initialization if they are used in an array of std::complex types.


typedef ap_fixed<DIN_W, 1, AP_TRN, AP_SAT> coeff_t; // MUST have IW >= 1
std::complex<coeff_t> twid_rom[REAL_SZ/2] = {{ 1, -0 },{ 0.9,-0.006 }, etc.}

The initialization values must first be cast to std::complex:


typedef ap_fixed<DIN_W, 1, AP_TRN, AP_SAT> coeff_t; // MUST have IW >= 1
std::complex<coeff_t> twid_rom[REAL_SZ/2] = {std::complex<coeff_t>( 1, -0 ), 
std::complex<coeff_t>(0.9,-0.006 ),etc.}