Useful IEEE Functions

Practical VHDL functions from std_logic_1164, numeric_std, std.env, textio and math_real.

Why this chapter

IEEE libraries do more than declare std_logic or unsigned. They also provide functions that make VHDL safer, clearer and easier to test.

The goal is not to memorize everything. The goal is to know which function to use in common situations.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

Keep this simple rule: for arithmetic, use numeric_std. Avoid old non-standard packages such as std_logic_arith, std_logic_unsigned or std_logic_signed.

`std_logic_1164`

The std_logic_1164 package defines std_logic, std_logic_vector and functions related to 9-state logic values.

Function	Use
`rising_edge(i_clk)`	Detect a rising clock edge
`falling_edge(i_clk)`	Detect a falling edge
`Is_X(w_bus)`	Detect an unknown value (`'U'`, `'X'`, etc.)
`To_01(w_bus)`	Map a vector back to `0`/`1` values
`to_hstring(w_bus)`	Print a vector in hexadecimal during simulation

Function	Use
`to_unsigned(i, n)`	Convert a natural integer to `unsigned(n-1 downto 0)`
`to_signed(i, n)`	Convert a signed integer to `signed(n-1 downto 0)`
`to_integer(x)`	Convert `unsigned` or `signed` to `integer`
`resize(x, n)`	Adapt the width of an `unsigned` or `signed` value
`shift_left(x, n)`	Shift left
`shift_right(x, n)`	Shift right
`rotate_left(x, n)`	Rotate left
`rotate_right(x, n)`	Rotate right

library STD;
use STD.TEXTIO.ALL;
 
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
 
file f_vectors : text open read_mode is "vectors.txt";
variable v_line : line;
variable v_data : std_logic_vector(7 downto 0);
variable v_ok   : boolean;
 
readline(f_vectors, v_line);
hread(v_line, v_data, v_ok);
 
assert v_ok
  report "Invalid stimulus line"
  severity error;

Need	Typical function
Detect an edge	`rising_edge`
Convert an integer to a bus	`to_unsigned`, `to_signed`
Convert a numeric bus to integer	`to_integer`
Change width	`resize`
Print a bus in hexadecimal	`to_hstring`
Read a hexadecimal vector in simulation	`hread`
End a testbench	`finish`
Generate random stimuli	`uniform`

Useful IEEE Functions

Why this chapter

`std_logic_1164`

Clock edges

Detect unknowns in simulation

`numeric_std`

Clean conversions

`resize` and result width

Display functions

`std.env`

`textio`

`math_real`

Key takeaways

Useful IEEE Functions

Why this chapter

std_logic_1164

Clock edges

Detect unknowns in simulation

numeric_std

Clean conversions

resize and result width

Display functions

std.env

textio

math_real

Key takeaways

`std_logic_1164`

`numeric_std`

`resize` and result width

`std.env`

`textio`

`math_real`