Outputting Don’t Care States in Hardware Programming: A VHDL Guide

In digital logic design, especially when using Hardware Description Languages (HDLs) like VHDL, the concept of “don’t care” states is crucial for optimization. These states, when properly utilized, allow synthesis tools to generate more efficient hardware by reducing the complexity of logic circuits. This is particularly relevant in scenarios where certain input conditions are guaranteed not to occur, or when the output is irrelevant for specific input combinations. A common example is in register file design, where write operations only depend on the write_enable signal; the write_address and write_data are inconsequential when writing is disabled. Effectively communicating these “don’t care” conditions to synthesis tools is key to leveraging their optimization capabilities.

But how can a hardware programmer effectively specify these “don’t care” values in VHDL to unlock these potential optimizations? This article explores several methods, examining their pros and cons to guide you in making the most appropriate choice for your hardware designs.

Methods to Specify “Don’t Care” in VHDL

There are a few approaches to representing “don’t care” conditions in VHDL, each with subtle differences in how they are interpreted by synthesis and simulation tools. Let’s delve into the most common methods:

1. Leaving Signals Unassigned

One seemingly straightforward approach is to simply not assign a value to a signal under specific conditions. The idea is that if a signal isn’t explicitly driven, the synthesis tool might infer a “don’t care.”

Pros:

• Simplicity: It can be conceptually simple to implement by omitting assignments in certain branches of your code.

Cons:

• Ambiguity and Simulation Issues: In simulation, unassigned signals can lead to unpredictable behavior, often being interpreted as ‘U’ (uninitialized). This can mask potential design flaws during verification.
• Limitations with Constants: This method falls short when defining constants, particularly of record types. VHDL typically requires all members of a record constant to be fully specified, making it impossible to leave fields unassigned directly.
• Synthesis Tool Dependency: The interpretation of unassigned signals as “don’t care” is not guaranteed across all synthesis tools. Some tools might not aggressively optimize in these scenarios, potentially missing optimization opportunities.

2. Utilizing ' - ' (Don’t Care Value from std_logic_1164)

The std_logic_1164 package, a cornerstone of VHDL for digital design, defines a specific value ' - ' (dash) explicitly designated as “Don’t care” for the std_ulogic and std_logic types. This appears to be the semantically correct way to represent a “don’t care” state in VHDL.

Pros:

• Semantic Clarity: Using ' - ' explicitly signals the designer’s intent of a “don’t care” condition. This is the most semantically accurate representation according to the VHDL standard.
• Potential for Optimization: Synthesis tools that correctly interpret the ' - ' value can leverage this information for more aggressive logic minimization, potentially leading to simpler and faster hardware.

Cons:

• Limited Industry Usage: Despite being semantically correct, the explicit use of ' - ' is not as widespread in practical VHDL codebases as one might expect. This could be due to historical reasons, toolchain variations, or a lack of awareness.
• Simulation Behavior: While semantically a “don’t care,” simulators might treat ‘-‘ somewhat differently than synthesis tools. It’s important to verify tool-specific behavior.

3. Employing ' X ' (Forced Unknown Value)

In simulation, particularly with tools like Modelsim, the value 'X' is often used to represent undefined or unknown signals. While primarily intended for simulation and debugging, one might consider using 'X' to represent “don’t care” in the hope that synthesis tools will interpret it similarly.

Pros:

• Simulation Visibility: 'X' clearly highlights uninitialized or conflicting signal states during simulation, which can be helpful for debugging.

Cons:

• Semantic Misinterpretation: 'X' fundamentally means “unknown” or “conflicting,” not explicitly “don’t care.” Relying on synthesis tools to interpret 'X' as “don’t care” is semantically incorrect and might not be consistently supported.
• Synthesis Tool Behavior: Synthesis tools are not guaranteed to treat 'X' as “don’t care.” They might propagate 'X' values through logic, potentially leading to unexpected or pessimistic synthesis results. It’s generally not recommended to use 'X' for specifying “don’t care” in synthesizable VHDL.

Code Example and Clarification

Consider the provided VHDL code snippet, which effectively uses ' - ' to initialize “don’t care” signals:

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

package mytypes is
    type control_signals_t is record
        write_enable  : std_logic;
        write_address : std_ulogic_vector(3 downto 0);
        read_address  : std_ulogic_vector(3 downto 0);
    end record;

    -- All members of this constant must be fully specified.
    -- So it's not possible to simply not assign a value.
    constant CONTROL_NOP : control_signals_t := (
        write_enable  => '0',
        write_address => (others => '-'),
        read_address  => (others => '-')
    );
end package;

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
library cfx;
use cfx.mytypes.all;

entity control_unit is
    port (
        instruction : in  std_ulogic_vector(15 downto 0);
        write_data  : out std_ulogic_vector(15 downto 0);
        ctrl        : out control_signals_t
    );
end entity;

architecture rtl of control_unit is
begin
    decode_instruction : process (instruction) is
    begin
        -- Set sensible default values that do nothing.
        -- Especially all "write_enable" signals should be '0'.
        -- Everything else is mostly irrelevant (don't care).
        ctrl <= CONTROL_NOP;

        if instruction(15 downto 12) = "1100" then
            -- Load 8 bit of data into the register file
            ctrl.write_enable  <= '1';
            ctrl.write_address <= instruction(11 downto 8);

        elsif instruction(15 downto 12) = "1101" then
            -- Load 8 bit of data into the register file
            ctrl.write_enable  <= '1';
            ctrl.write_address <= instruction(3 downto 0);
        end if;
    end process decode_instruction;


    write_data <= (others => '0'); -- Dummy output
end architecture;

In this example, the CONTROL_NOP constant initializes write_address and read_address with (others => '-'). This is a clear and explicit way to indicate that when write_enable is '0', the values of write_address and read_address are “don’t care.” When synthesized, a tool that recognizes ' - ' should be able to optimize the logic driving these signals, potentially leading to a simpler multiplexer implementation as hypothesized in the original question.

Conclusion

While multiple approaches exist, using ' - ' from std_logic_1164 is the most semantically accurate and potentially most effective method for specifying “don’t care” states in VHDL for synthesis optimization. While it might not be universally practiced, its explicit nature clearly communicates design intent to both synthesis tools and other engineers reading the code. Avoiding unassigned signals for “don’t care” is generally advisable due to simulation ambiguities, and using 'X' for this purpose is semantically incorrect and unreliable for synthesis.

For robust and optimized hardware designs, especially when aiming for clarity and leveraging the full potential of synthesis tools, explicitly using ' - ' to denote “don’t care” conditions in VHDL is the recommended best practice. Always verify the specific behavior of your chosen synthesis and simulation tools to ensure they correctly interpret and optimize based on your “don’t care” specifications.