• Logisim home page
• Guide to Being a Logisim User
  • Beginner's tutorial
    • Step 0: Orienting yourself
    • Step 1: Adding gates
    • Step 2: Adding wires
    • Step 3: Adding text
    • Step 4: Testing your circuit
  • Libraries and attributes
    • The explorer pane
    • The attribute table
    • Tool and component attributes
  • Subcircuits
    • Creating circuits
    • Using subcircuits
    • Debugging subcircuits
    • Logisim libraries
  • Wire bundles
    • Creating bundles
    • Splitters
    • Wire colors
  • Combinational analysis
    • Opening Combinational Analysis
    • Editing the truth table
    • Creating expressions
    • Generating a circuit
  • Menu reference
    • The File menu
    • The Edit menu
    • The Project menu
    • The Simulate menu
    • The Window and Help menus
  • Memory components
    • Poking memory
    • Pop-up menus and files
    • Hex editor
  • Logging
    • The Selection tab
    • The Table tab
    • The File tab
  • Application preferences
    • The Template tab
    • The Tools tab
    • The International tab
    • The Experimental tab
    • The command line
  • Project options
    • The Canvas tab
    • The Simulation tab
    • The Toolbar tab
    • The Mouse tab
  • Value propagation
    • Gate delays
    • Oscillation errors
    • Shortcomings
  • JAR libraries
    • Gray Code Incrementer
    • Library Class
    • Simple Gray Code Counter
    • Gray Code Counter
    • Guidelines
  • About the program
    • GNU General Public License
• Library Reference
  • Base library
    • Poke Tool
    • Edit Tool
    • Select Tool
    • Wiring Tool
    • Text Tool
    • Menu Tool
    • Splitter
    • Pin
    • Probe
    • Clock
    • Text
  • Gates library
    • Constant
    • NOT Gate
    • Buffer
    • AND/OR/NAND/NOR Gate
    • XOR/XNOR/Even/Odd Gate
    • Controlled Buffer/Inverter
  • Plexers library
    • Multiplexer
    • Demultiplexer
    • Decoder
    • Priority Encoder
    • Bit Selector
  • Arithmetic library
    • Adder
    • Subtractor
    • Multiplier
    • Divider
    • Negator
    • Comparator
    • Shifter
  • Memory library
    • D/T/J-K/S-R Flip-Flop
    • Register
    • Counter
    • Shift Register
    • Random
    • RAM
    • ROM
  • Input/Output library
    • Button
    • Joystick
    • Keyboard
    • LED
    • 7-Segment Display
    • Hex Digit Display
    • LED Matrix
    • TTY
  • Legacy library
    • Logisim 1.0 D/J-K Flip-Flop
    • Logisim 1.0 8-Bit Register

Oscillation errors

The propagation algorithm, which normally works silently without any problems, will become very visible when you create a circuit that oscillates.

This circuit is currently in a stable condition. But if you change the input to 1, the circuit will effectively enter an infinite loop. After a while, Logisim will simply give up and show an "Oscillation apparent" message telling you that it believes that the circuit is oscillating.

It will display the values it has at the time it gives up. These values will look wrong - in this screen shot, the AND gate is emitting 1 although one of its inputs is 0, but it could be that the NOT gate has a 1 input and a 1 output.

Logisim helpfully circles in red each location that seems to be involved in the oscillation. If an involved point lies within a subcircuit, Logisim will draw that subcircuit's outline in red.

When Logisim detects oscillation, it shuts down all further simulation. You can re-enable simulation using the Simulate menu's Simulation Enabled option.

Logisim detects oscillation using a fairly simple technique: If the circuit simulation seems to many iterations, then it will simply give up and report oscillation. (The points it identifies as being involved are those that were touched in the last 25% of the iterations.) Thus, it could erroneously report oscillation, particularly if you are working with an exceptionally large circuit; but it would be one that is larger than any I have built using Logisim. In any case, if you are confident that the reporting is in error, you can configure the number of iterations completed before oscillation occurs via the Project Options window's Simulation tab.

Next: Shortcomings.