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Race Conditions

  Race conditions may occur when two or more signals change value simultaneously. As an example, consider a NOR-based (reset dominant) SR latch, where

$$\begin{array}{ccc} S=1 & & R=1 \\ & \downarrow & \\ S =0 & & R=0 \end{array}$$

(You may recall that S=R=1 is illegal for the SR latch.) The question is:

$$\begin{array}{c} Q \\ \downarrow \\ ? \end{array}$$

The point is that signals can never exactly change value simultaneously, so this means that the value of Q is unpredictable:

R=0 before S=0 $\to$ Q=1
S=0 before R=0 $\to$ Q=0

We will see other examples of race conditions later.

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