Is there a way to print the values of a signal to a file from a modelsim simulation?

Question

I need to get the values of several signals to check them against the simulation (the simulation is in Matlab). There are many values, and I want to get them in a file so that I could run it in a script and avoid copying the values by hand.

Is there a way to automatically print the values of several signals into a text file?

(The design is implemented in VHDL)

Solution

First make functions that convert std_logic and std_logic_vector to string like:

function to_bstring(sl : std_logic) return string is
  variable sl_str_v : string(1 to 3);  -- std_logic image with quotes around
begin
  sl_str_v := std_logic'image(sl);
  return "" & sl_str_v(2);  -- "" & character to get string
end function;

function to_bstring(slv : std_logic_vector) return string is
  alias    slv_norm : std_logic_vector(1 to slv'length) is slv;
  variable sl_str_v : string(1 to 1);  -- String of std_logic
  variable res_v    : string(1 to slv'length);
begin
  for idx in slv_norm'range loop
    sl_str_v := to_bstring(slv_norm(idx));
    res_v(idx) := sl_str_v(1);
  end loop;
  return res_v;
end function;

Using the bit-wise format has the advantage that any non-01 values will show with the exact std_logic value, which is not the case for e.g. hex presentation.

Then make process that writes the strings from std_logic and std_logic_vector to file for example at rising_edge(clk) like:

library std;
use std.textio.all;
...
process (clk) is
  variable line_v   : line;
  file     out_file : text open write_mode is "out.txt";
begin
  if rising_edge(clk) then
    write(line_v, to_bstring(rst) & " " & to_bstring(cnt_1) & " " & to_bstring(cnt_3));
    writeline(out_file, line_v);
  end if;
end process;

The example above uses rst as std_logic, and cnt_1 and cnt_3 as std_logic_vector(7 downto 0). The resulting output in "out.txt" is then:

1 00000000 00000000
1 00000000 00000000
1 00000000 00000000
0 00000000 00000000
0 00000001 00000011
0 00000010 00000110
0 00000011 00001001
0 00000100 00001100
0 00000101 00001111
0 00000110 00010010