Design Guidelines for Logisim: Bad Practices to Avoid

CS3410 Spring 2013

Bad design #1: Unlabeled circuit diagrams

• A circuit with no labels is like code with no comments.

For both your own understanding and our grading, it is easier to follow your logic if you specify what the subcircuits are and what they're used for.

Bad design #2: Too many muxes

• Having too many muxes are both inefficient and potentially redundant.

As in the case with V in lab1, there is no need for a 16 bit mux in ALU32. The cases in which V appears was few and the majority of the time it should be 0 instead. Instead of selecting V on ALU opcode, think of other simpler options.

Bad design #3: Multibit constant

• Constants should not be used to extend a number if a bit extender can be used instead.

Bit extender is neater and easier to understand.

Bad idea: Using a splitter and constant to perform extension.
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It is better design to use as few constants as possible.

Bad design #4: Muxing a signal and its inverse

• Muxing between a signal and its inverse is redundant.

This can easily be replaced with an XOR gate.

Bad idea: Use a multiplexor with inverses as inputs.
wrong	right
These almost always reduce to just an AND gate, without the constant. Or in the worst case, a no-op. The circuits on the left and right are equivalent.

Bad design #5: Improper use of constants

• In general, you should not need any constants in your circuits.

The only exceptions are for (a) providing inputs to a sub-circuit that has more inputs than you need, or (b) making your circuit look slightly simpler in certain cases. Any time you feel like putting down a constant, just ask "can I just optimize this away by using a different gate?"

Bad idea: Use a multiplexor with constants as inputs.
wrong	right
These almost always reduce to just an AND gate, without the constant. Or in the worst case, a no-op.

Bad design #6: Repetitive use of input to both data and control

• There is no need to use the same signal for data and control of the same mux.

This case is very similar to the case above. It can easily be replaced with logic gates.

Bad idea: Use a multiplexor with same signal as input and control.
wrong	right
These circuits are equivalent. The control is just a case of OR.

Bad design #7: Useless sub-circuits

• If a sub-circuit has only one component, it probably doesn't deserve to be a sub-circuit.

Instead, just use the component itself. An exception to this rule is the "bit reverser" circuit that many of you built (with no components, just splitters and wires), since it takes up so much screen space.

Annoyance: unnecessary and distracting sub-circuits
wrong	also wrong
These sub-circuits are just re-packaging and re-naming already existing Logisim primitives. Just use the primitive instead.

Bad design #8: Not testing completely

• Make sure the circuit you turned in does not have any red wires and can pass all your own test vectors. If your circuit is supposed to work on signed two's-complement numbers, please test it with both positive and negative numbers.

At minimum, do positive+positive, positive+negative, negative+negative, and negative+positive. And try a couple of each combination, too.

Logisim quirk: data flow

This is a minor point. But you have probably noticed that logisim likes data to flow left-to-right on the screen. For example, the default orientation of primitives are all left-to-right, and multiplexors can't be rotated at all. Many of you displayed an amazing stubbornness by having your circuits go in other directions.

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Don't fight it.