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<title>Gate delays</title>
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<h1>Gate delays</h1>

<p>As an example of the level of sophistication of Logisim's algorithm,
consider the following circuit.</p>
<center><img src="../../../../en/img-guide/prop-const0.png" width="171" height="55"></center>
<p>This "obviously" always outputs 0. But NOT gates don't react
instantaneously to their inputs in reality, and neither do they in
Logisim. As a result, when this circuit's input changes from 0 to 1, the
AND gate will briefly see two 1 inputs, and it will emit a 1 briefly.
You won't see it on the screen. But the effect is observable when we
use the AND gate's output as an input into the clock of a D flip-flop.</p>
<center><img src="../../../../en/img-guide/prop-using-const0.png" width="231" height="68"></center>
<p>Poking the 0 input to become 1 leads to an instantaneous 1 going
into the D flip-flop, and thus the flip-flop's value will toggle
every time the circuit input goes from 0 to 1.</p>

<p>Every component has a delay associated with it. More sophisticated
components built into Logisim tend to have larger delays, but
these delays are somewhat arbitrary and may not reflect reality.</p>

<p>From a technical point of view, it is relatively easy to deal with
this level of sophistication in a single circuit.  Dealing with gate
delays well across subcircuits, though, is a bit more complex;
Logisim does attempt to address this correctly by placing all primitive
component's propagation values into a single schedule regardless of the
subcircuit in which the component lies.</p>

<p>(Via the <a href="../opts/index.html">Project Options</a> window's
<a href="../opts/simulate.html">Simulation tab</a>, you can configure Logisim
to add a random, occasional delay to a component's propagation. This is intended
to simulate the unevenness of real circuits. In particular, an R-S latch built
using two NOR gates will oscillate without this randomness, as both gates will
process their inputs in lockstep. This randomness is disabled by default.)</p>

<p>Note that I'm stopping short of saying that Logisim
always addresses gate delays well.  But at least it tries.</p>

<p><strong>Next:</strong> <a href="oscillate.html">Oscillation errors</a>.</p>

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