How to write an integer to stdout as hexadecimal in VHDL?

Question

I can print an integer as decimal to stdout with:

library std;
use std.textio.all;

entity min is
end min;

architecture behav of min is
begin
    p         


        

Answer 1

There is no standard library implementation, but for example our PoC-Libary has several formatting function in the PoC.Strings package. On top of that, we have a to_string(...) function, which accepts a format character like h for hexadecimal outputs.

How to write such an integer to hex conversion?

1. Convert the INTEGER into a binary representation
2. Group the binary value into 4-bit groups
3. translate each group into an integer/alpha in range 0..F
4. prepend 0x if wished

So here is a wrapper to convert the integer to a binary representation:

-- format a natural as HEX string
function raw_format_nat_hex(Value : NATURAL) return STRING is
begin
  return raw_format_slv_hex(std_logic_vector(to_unsigned(Value, log2ceil(Value+1))));
end function;

And now the grouping and transformation

-- format a std_logic_vector as HEX string
function raw_format_slv_hex(slv : STD_LOGIC_VECTOR) return STRING is
  variable Value                : STD_LOGIC_VECTOR(4*div_ceil(slv'length, 4) - 1 downto 0);
  variable Digit                : STD_LOGIC_VECTOR(3 downto 0);
  variable Result               : STRING(1 to div_ceil(slv'length, 4));
  variable j                    : NATURAL;
begin
  Value := resize(slv, Value'length);
  j             := 0;
  for i in Result'reverse_range loop
    Digit       := Value((j * 4) + 3 downto (j * 4));
    Result(i)   := to_HexChar(unsigned(Digit));
    j           := j + 1;
  end loop;
  return Result;
end function;

-- convert an unsigned value(4 bit) to a HEX digit (0-F)
function to_HexChar(Value : UNSIGNED) return CHARACTER is
  constant HEX : STRING := "0123456789ABCDEF";
begin
  if (Value < 16) then
    return HEX(to_integer(Value)+1);
  else
    return 'X';
  end if;
end function;

-- return TRUE, if input is a power of 2
function div_ceil(a : NATURAL; b : POSITIVE) return NATURAL is  -- calculates: ceil(a / b)
begin
  return (a + (b - 1)) / b;
end function;

-- return log2; always rounded up
function log2ceil(arg : positive) return natural is
  variable tmp : positive;
  variable log : natural;
begin
  if arg = 1 then   return 0; end if;
  tmp := 1;
  log := 0;
  while arg > tmp loop
    tmp := tmp * 2;
    log := log + 1;
  end loop;
  return log;
end function;

Note: These functions do not prepend 0x.