VHDL Attributes: 'range, 'length, 'high, and 'low

Learn how VHDL attributes make reusable, width-independent code and when to use 'range, 'length, 'high, 'low, and synthesis attributes.

Why Attributes Matter

An attribute asks a type, signal or range for information. The most useful ones avoid hardcoded sizes.

signal r_data : std_logic_vector(15 downto 0);
 
constant c_WIDTH : natural := r_data'length;

If r_data changes from 16 to 32 bits, c_WIDTH follows automatically.

Range Attributes

Example:

signal r_data : std_logic_vector(7 downto 0);

For this signal:

• r_data'length is 8;
• r_data'high is 7;
• r_data'low is 0;
• r_data'range is 7 downto 0.

Robust Loops

Instead of writing a loop tied to a fixed size:

for i in 0 to 7 loop
  r_next(i) <= i_data(i);
end loop;

use the signal range:

for i in i_data'range loop
  r_next(i) <= i_data(i);
end loop;

The code stays correct if i_data changes width or if the range direction changes.

Conversions and Sizes

Attributes are also useful for conversions.

signal r_count : unsigned(7 downto 0);
signal w_vec   : std_logic_vector(7 downto 0);
 
w_vec <= std_logic_vector(resize(r_count, w_vec'length));

Here, the target width comes from w_vec. The code does not repeat 8.

rising_edge Instead of 'event

The 'event attribute indicates that a signal changed value. You may encounter this style:

if i_clk'event and i_clk = '1' then
  -- ...
end if;

In modern VHDL, prefer:

if rising_edge(i_clk) then
  -- ...
end if;

rising_edge directly expresses the intent and handles std_logic transitions more cleanly.

Synthesis Attributes

Some attributes are interpreted by synthesis tools. They can guide the tool, but they should remain exceptional.

attribute keep : string;
attribute keep of w_debug : signal is "true";

This example asks the tool to preserve an intermediate signal. Depending on the tool, the exact attribute name and accepted values can vary.

Best Practices

• Use 'length for size conversions.
• Use 'range to iterate over a bus.
• Prefer rising_edge(i_clk) over a condition based on 'event.
• Avoid magic numbers in bus widths.
• Document every synthesis attribute: it is often tool-dependent.

Key Point

Attributes make VHDL more robust. They let code follow the real size of signals instead of copying the same constants everywhere.

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