Write to File in VHDL using TextIO Library

When you simulate a design in VHDL it is very useful to have the possibility to save some simulation results. For example, if you need to compare the simulation results with reference test vector, or simply to document the simulation in a test report.

In VHDL, there are predefined libraries that allow the user to write to an output ASCII file in a simple way. The TextIO library is a standard library that provides all the procedure to read from or write to a file.

It is clear that these procedures cannot be used in a synthesizable RTL VHDL code, I mean no file handling possibility is present into a silicon device using simple RTL VHDL code, but they are very useful in test bench design.

 

There are several different ways to open a file and write to it.

In VHDL, File are handled as array of line an example of VHDL syntax to write to file is:

Declare and Open file in write mode:
file file_handler     : text open write_mode is “filename.dat”;
Variable row          : line;
Variable v_data_write : integer;


Write value to line
write(row, v_data_write);

Write line to the file
writeline(file_handler ,row);

The last action is used to write the desired value from the line to the file. If v_data_read is an integer value, the “write” procedure will try to write an integer value to the output line, if it is a real value, the procedure will try to write a real number and so on.

In this bunch of VHDL code, the file is opened during the file declaration on row 2.

A complete example of a process that write to a file is given below:

-- procedure WRITE(L : inout LINE; VALUE : in integer; JUSTIFIED: in SIDE := right; FIELD: in WIDTH := 0);
.
.
signal o_valid        : std_logic;
signal o_add          : std_logic_vector(7 downto 0);
.
.
.
p_dump  : process(i_rstb,i_clk)

file test_vector      : text open write_mode is "output_file.txt";
variable row          : line;

begin

  
  if(i_rstb='0') then

  ------------------------------------
  elsif(rising_edge(i_clk)) then
    
    if(o_valid = '1') then
    
      write(row,o_add, right, 15);
      
      write(row,conv_integer(o_add), right, 15);

      hwrite(row,o_add, right, 15);

      hwrite(row,"00000000"&o_add, right, 15);
      
      writeline(test_vector,row);
    
    end if;
  end if;
end process p_dump;

The code above implements a process that at each clock cycle to an output file the following value:

• Column 1: 8-bit value
• Column 2: integer values;
• Column 3: 8-bit hexadecimal value
• Column 4: 16-hexadecimal value

Data is written if control signal “o_valid” is ‘1’.

This code writes all the values right justified and each column contains 15 character

The output file will have the following format:

00000011              3             03           0003
00001010             10             0A           000A
00000010              2             02           0002
01011010             90             5A           005A

Here below there are the procedures you can find in IEEE TextIO standard library to write to a file different type of values:

procedure WRITELINE(file f : TEXT; L : inout LINE);

procedure TEE (file F :       TEXT; L : inout LINE);

procedure WRITE(L : inout LINE; VALUE : in bit;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in bit_vector;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in BOOLEAN;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in character;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in integer;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in real;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0;
    DIGITS: in NATURAL := 0);

procedure WRITE (L      : inout LINE; VALUE : in REAL;
         FORMAT : in    STRING);

procedure WRITE(L : inout LINE; VALUE : in string;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0);

procedure WRITE(L : inout LINE; VALUE : in time;
    JUSTIFIED: in SIDE := right;
    FIELD: in WIDTH := 0;
    UNIT: in TIME := ns);

alias SWRITE is WRITE [LINE, STRING, SIDE, WIDTH];
alias STRING_WRITE is WRITE [LINE, STRING, SIDE, WIDTH];
alias BWRITE is WRITE [LINE, BIT_VECTOR, SIDE, WIDTH];
alias BINARY_WRITE is WRITE [LINE, BIT_VECTOR, SIDE, WIDTH];

procedure OWRITE (L         : inout LINE; VALUE : in BIT_VECTOR;
        JUSTIFIED : in    SIDE := right; FIELD : in WIDTH := 0);
alias OCTAL_WRITE is OWRITE [LINE, BIT_VECTOR, SIDE, WIDTH];
procedure HWRITE (L         : inout LINE; VALUE : in BIT_VECTOR;
        JUSTIFIED : in    SIDE := right; FIELD : in WIDTH := 0);
alias HEX_WRITE is HWRITE [LINE, BIT_VECTOR, SIDE, WIDTH];

 

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