MINI LESSON 1 – INTRO TO SEQUENTIAL CIRCUITS
SEQUENTIAL VS
COMBINATIONAL CIRCUITS
Combinational circuits are
circuits where the inputs rely strictly on the values of the inputs. Every circuit in the first Digital Design
unit was a combinational circuit.
In sequential circuits, the outputs rely
on the value of the inputs but also on their own previous state(s). This is because the output of the circuit
is fed back (feedback) in as an input to at least a section of the circuit.
FEEDBACK
Feedback, or a feedback loop, is created when one (or more) of the outputs of a
circuit is fed back to the circuit as one of the inputs. This means that the value of the output is
dependent on the inputs and its own previous value.
PROPAGATION
DELAY
So far, we
have worked with the idea that gates work instantaneously. However, there is a small amount of time
that is required for the gate’s outputs to reflect a changed input. This time is called the propagation delay.
To give you
an idea of actual time, the propagation delay for a NOT gate (7404) is stated
in various datasheets to be between 5 and 20 nanoseconds.
RACE CONDITION
A race condition is where the output of
a circuit is dependent on the propagation of the input signals through the
circuit. This can lead to errors or
unexpected results. Other times, it is
not noticeable. Race conditions can
occur in both combinational and sequential circuits. Here is an example:
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EXAMPLE
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