Constant

Found in Wiring Library

Use a constant when a circuit calls for a fixed, known value. You can specifify a value for each constants in binary, hex, or decimal, and the constant will hold that value during the circuit simulation.

See Also:

• Live Circuit: Constants
• Live Circuit: Fizz-Buzz

Constants can be unintuitive at first. But if you are familiar with programming, think of constants in Logisim circuits as playing a similar role as "literal values" like 6 and 12 in a statement like y = 12 + x*6. They are fixed values, not inputs (like x) or outputs (like y).

It's a common mistake to use an Input Pin when a constant would be better. An Input Pin takes on different values, depending on user interaction, and it resets to zero when the simulation is reset. The value of an Input Pin is not saved with a project, but instead resets to zero whenever a project is opened again. By contrast, a Constant has a fixed value that does not change during a simulation and is saved with project file.

Similarly, here are some other common mistakes, and how to fix them:

• Mistake: Using an Input Pin, with a label nearby saying "Don't touch this input, it must be zero or the circuit won't work."
  Fix: Use a constant with value 0.

• Mistake: Using an Input Pin and expecting it to be zero at all times, and hoping the user never pokes the input.
  Fix: Use a constant with value 0.

• Mistake: Using an Input Pin, with a label nearby saying "Before simulating this circuit, please put hex value 4c in this input, otherwise the circuit won't work."
  Fix: Use a constant with hex value 4c. Or use whatever number base is most convenient.

• Mistake: Cleverly using 1-bit Input Pin, wired to both inputs of a XOR gate, followed by a NOT gate, knowing that anything XOR'ed with itself will result in a 0, so the NOT gate will always output 1.
  Fix: Use a 1-bit constant with value 1.

• Mistake: Using a 1-bit constant 0, followed by a NOT gate, to obtain the value 1.
  Fix: Use a 1-bit constant with value 1.

• Mistake: Using the power or ground components from the Analog Library with components expecting digital or boolean inputs.
  Fix: Use constants instead. Although the power and ground components work identically to constant 1 and constant 0, they have more limited options, and they are intended for use with other Analog Library components or places where defined voltage devel is needed for "power", rather than a digital or Boolean true/false value.

Ports

Each constant has a single port, which constantly carries the specified value.

Keyboard Shortcuts

• Arrow keys change the Facing property.
• Typing hex digits (0-9, a-f) changes the Value property. Note that any value typed this way is interpreted as hex, regardless of the constant's Radix property. For example, if an 8-bit constant is set to display with unsigned decimal for Radix, typing f f will change its value to 255, since 255 decimal is 0x80 hex.
• Typing a number (1-32) while holding the option key (⌥) or Alt key changes the Data Bits property.

Properties

Facing

The side where the port should be.

Radix

The radix for displaying the value: binary, octal, hexadecimal, signed decimal, or unsigned decimal.

Data Bits

The number of bits for the constant value. The underlying binary representation of the value must fit within this number of bits.

Value

The value for the constant, specified in the chosen radix. The allowable range of values depends on the radix and data bits chosen. For example, the range for 8-bit signed decimal is -128 to +127, while the range for 8-bit unsigned decimal is 0 to 255.

Poke Tool Behavior

None - constants do not respond to being poked.

FPGA Synthesis

Supports VHDL and Verilog synthesis.