Input & Output Pin

Found in Wiring Library

Pins act as inputs to, or outputs from, a circuit. Input pins have squared corners, output pins are rounded.

See Also:

• Live Circuit: Pins

Pins are interactive: you can poke an input pins to change the value they send into your circuit, and output pins can't be poked but instead always display the value coming from your circuit, or if there is no value to display, output pins show a 'z', the standard notation for "floating" or "no specific value". When used interactively, a 1-bit input pin behaves much like a Button, and a 1-bit output pin behaves similarly to an LED. Unlike Buttons and LEDs, the colors of pins can't be customized. But pins can carry multiple bits, and display their values numerically in your choice of format (binary, signed decimial, hexadecimal, etc.), features not available for Button and LED.

Pins enable subcircuits: When one circuit is embedded as a subcircuitwithin another, pins are how values traverse between the outer and the inner circuits. The outer circuit sends data into the inner circuit's input pins, and inner circuit's output pins feed data back to the outer circuit. No other component has this ability: a Button inside the inner circuit, for example, can't be poked or driven by signals from the outer circuit, and an LED inside the inner circuit can't affect or be seen from the outer circuit. (But see dynamic subcircuit appearance for an advanced technique allowing more flexibility when designing subcircuits.)

Ports

Each pin has a single port. For input pins, the port carries the input value. For output pins, the signal at this port becomes the output.

Appearance

Normally, 1-bit pins show color-coded binary values, '0' or '1'. Pins show 'z' to indicate a floating value (for example, if an output pin isn't connected to a wire, or is connected to a wire that doesn't carry any value). In cases of errors, pins become red, and output pins show 'E'.

Multi-bit pins show values in numeric form, but don't show colors. They can be configured to show binary, hexadecimal, octal, signed decimal, or unsigned decimal. The symbols 'z' and 'E' are also used to indicate floating values and errors for multi-bit pins.

In printer view, or when exporting to image format, pins normally display their width, instead of showing a value.

Interaction

Input pins are interactive components. For a standalone or top-level circuit, use the Poke Tool to change the value of an input pin: each poke with toggle between '0' and '1', or if the Radix is not binary, poke will cycle through the possible values or open an input box to enter a value.

When viewing the state of a subcircuit, part of some larger circuit simulation, input pins values are "tied" to the outer circuit, so the poke tool can't change the values.

When viewing an "inner" simulation, if you poke an input pin, a notice will appear warning that the pins are "tied," or linked, to the outer circuit and can't be changed. The notice window also gives you the chance to break this link by creating a new standalone simulation where you will be able to modify the input pin values. See debugging subcircuits for more info.

Advanced Input Behaviors

When the Behavior property for an input pin is set to Simple (the default), the pin will normally send either a '0' or '1' into a circuit, at all times (or, for a multi-bit input, a pattern of zeros and ones). But this isn't quite true.

• For a top-level, standalone simulation: yes, the poke tool toggles the input pin value between '0' and '1', with no other choices.
• But when acting as a subcircuit, the value of the input pin comes from the outer circuit, and this value could be something other than '0' or '1'.

An outer circuit might have a mistake causing an error signal to appear at the input pin. Or, a wire in the outer circuit that connects to the input pin might not be driven by any components, leaving the value floating, with a 'z' value appearing at the input pin. As a special case: if the outer circuit doesn't have any connections to the pin, logisim uses a default value of '0' for the pin. In other words, a Simple input pin will pass along a floating value from a connected wire in an outer circuit, but will never "create" a floating value via the poke tool or because there is no connected wire in the outer circuit.

This special case for Simple — using '0' for the value of a completely-unconnected subcircuit input pin — can be confusing, and can lead to subtle mistakes. If you accidentally miswire the outer circuit, or forget to connect something to the subcircuit input, you may not notice the mistake, as it will be treated as if it were '0'. This behavior is also inconsistent with reality. The advanced behaviors (described below) are a better option, and this special case was probably not the best idea. Nevertheless, the special case is retained for backwards compatibility with older logisim circuits.

The Behavior property allows other options than the Simple behavior described above:

• Tri-State behavior: The poke tool toggles between '0', '1', and 'z' (floating). And there is no special case: a tri-state input within a subcircuit takes on whatever value the outer circuit gives it, whether that is '0' or '1' (normal cases), 'E' (error cases), or 'z' (floating, when the pin is unconnected, or is connected to a wire that doesn't carry a value).
• Pull-Down behavior: Poke behaves like the Simple behavior, but when used in a subcircuit, all 'z' values are converted to '0'.
• Pull-Up behavior: Similar, but 'z' values are converted to '1'.

These three options match more closely the behavior of FPGA devices and other physical circuits.

Keyboard Shortcuts

When using Poke Tool: If an input pin has a single-character label, type that character to poke the pin and change it's value.

When using Edit Tool:

• Arrow keys change the Facing property.
• Arrow keys while holding the option key (⌥) or Alt key change the Label Location property.
• 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. By default, input pins face east, output pins face west.

Type

Whether pin accepts input (from user, or from an outer circuit in which this circuit is embedded as a subcircuit), or displays, or conveys output (to the user, or to an outer circuit in which this circuit is embedded as a subcircuit).

Data Bits

The number of bits for the value that the pin handles.

Behavior (only present for input pins)

Controls how an input pin handles floating values.

• Simple: Poke tool toggles between '0' and '1', or patterns of them. When used in a subcircuit, unconnected pins will default to '0', but otherwise pass along whatever value the connected wire carries.
• Tri-State: Poke tool toggles between '0', '1', and 'z', or patterns of them. When used in a subcircuit, unconnected pins are floating ('z'), and otherwise pass along whatever value the connected wire carries in the outer circuit.
• Pull Down and Pull Up: Poke tool toggles between '0' and '1', or patterns of them. When used in a subcircuit, unconnected pins or floating values on a connected wire in the outer circuit are converted to either '0' (for pull down) or '1' (for pull up), and otherwise pass along whatever value the connected wire carries. This does not affect the outer circuit.

Label

A name for the input or output.

Label Location

Location of the label relative to the component.

Label Font

Font for the label.

Radix

The number base used for displaying values, and for entering values for input pins.

Poke Tool Behavior

Clicking an input pin with the poke tool will change the value it contains.

• For 1-bit pins or when Radix is Binary: clicks toggle the bit it contains, or click then type '0' or '1'. If using Tri-State behavior, then 'z' is also allowed.
• If Radix is Hexadecimal: clicks cycle through '0' to 'F'. Or click then type a hex digit. If using Tri-State behavior, then 'z' is also allowed (in hex, 'z' is treated like binary 'zzzz', that is, four floating values).
• If Radix is Octal: clicks cycle through '0' to '7'. Or click then type an octal digit. If using Tri-State behavior, then 'z' is also allowed (in octal, 'z' is treated like binary 'zzz', that is, three floating values).
• If Radix is Signed Decimal or Unsigned Decimal: clicks open a dialog box where you can type a number.

  Output pins do not respond to being poked.

  FPGA Synthesis

  Supports VHDL and Verilog synthesis.