Implementing ROMs - 2022.1 English

Vitis High-Level Synthesis User Guide (UG1399)

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2022.1 English

Vitis HLS does not require that an array be specified with the static qualifier to synthesize a memory or the const qualifier to infer that the memory should be a ROM. Vitis HLS analyzes the design and attempts to create the most optimal hardware.

Important: Xilinx highly recommends using the static qualifier for arrays that are intended to be memories. As noted in Array Initialization, a static type behaves in an almost identical manner as a memory in RTL.
The const qualifier is also recommended when arrays are only read, because Vitis HLS cannot always infer that a ROM should be used by analysis of the design. The general rule for the automatic inference of a ROM is that a local (non-global), static array is fully written to before being read, and never written again. The following practices in the code can help infer a ROM:
  • Initialize the array as early as possible in the function that uses it.
  • Group writes together.
  • Do not interleave array(ROM) initialization writes with non-initialization code.
  • Do not store different values to the same array element (group all writes together in the code).
  • Element value computation must not depend on any non-constant (at compile-time) design variables, other than the initialization loop counter variable.

If complex assignments are used to initialize a ROM (for example, functions from the math.h library), placing the array initialization into a separate function allows a ROM to be inferred. In the following example, array sin_table[256] is inferred as a memory and implemented as a ROM after RTL synthesis.

#include "array_ROM_math_init.h"
#include <math.h>

void init_sin_table(din1_t sin_table[256])
 int i;
 for (i = 0; i < 256; i++) {
 dint_t real_val = sin(M_PI * (dint_t)(i - 128) / 256.0);
 sin_table[i] = (din1_t)(32768.0 * real_val);

dout_t array_ROM_math_init(din1_t inval, din2_t idx)
 short sin_table[256];
 return (int)inval * (int)sin_table[idx];
Tip: Because the sin() function results in constant values, no core is required in the RTL design to implement the sin() function.