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Combinational vs. Sequential Circuits

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(Combinational versus Sequential Circuits) Which of the fol¬lowing contain circuits that are likely to be combinational and which contain sequential circuits? Explain your rationale.

(a) A washing machine that sequences through the soak, wash, and spin cycles for preset periods of time.

(b) A circuit that divides two 2-bit numbers to yield a quotient
and a remainder.

(c) A machine that takes a dollar bill and gives three quarters, two dimes, and a nickel in change, one at a time through a
single coin change slot.

(d) A digital alarm clock that generates an alarm when a preset time has been reached.

(e) A circuit that takes as input two decimal numbers in the range from 0 to 9, outputs a 0 if they are different, and a 1 if they are identical.

(f) A circuit that turns on or off a hall light based on the configuration of two input switches. If both Switches are in the same position, the light is off. If they are in different positions the light is on.

(g) A circuit that takes a sequence of bits, one bit at a time, and outputs a 0 or 1 after each bit that indicates if the number of
is in the sequence seen so far is even or odd, respectively.

(h) A circuit with two binary inputs and four binary outputs that works as follows. The binary input indicates which of the four
outputs should be driven to a 1 with the other outputs set to 0.

Solution Preview

A combinational circuit's output does not depend on the previous output. It has no memory and if we know the current state there is no way to know what was the previous output state.

Sequential circuit's output can depend on the previous output state. This circuit has "memory" - it remembers at least the previous state. Actually, if we know one state we can theoretically trace the sequence of the output all the way to the initial state.

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A washing machine that sequences through the soak, wash, and spin cycles for preset periods of time

By the name we can see that the circuit is sequential - the circuit output (ie, the washing stage) depends on the previous state. It will not go to "wash" after "spin" state output , but it will if the previous state is "soak".
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A circuit that divides two 2-bit numbers to yield a quotient and a remainder.

Combinational circuit. the Output ...

Solution Summary

This solution explains combinational vs. sequential circuits.

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