Question

Consider a sequential circuit to control the washer:

The circuit receives 3 input signals: Clock, DoorOpen, and Start. It also maintains a state bit Wash.

If DoorOpen = 1, Wash should be set to 0.

Otherwise, if Start = 1, Wash should be set to 1 and if Start = 0, Wash should retain its value from previous cycle.

Make A truth table to compute the new value of Wash;

Make A circuit Diagram.

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How would the truth table look for this problem? I'm confused about how the state bit works with the table.

Here's my first attempt:

Clock	Door Open	Start	Current Wash	Next Wash
0	0	0	1	1
0	0	1	1	0
0	1	0	0	0
0	1	1	1	0
1	0	0	1	1
1	0	1	1	0
1	1	0	0	0
1	1	1	1	0

I don't know if this is right, let me know what is wrong with this.

Answer 1

Here one thing to note that what is the purpose of clock input, is it use as a power supply to the circuit or it is used as sequential clock, which is used to change the state of the output or it is just a input signal which is used as external input like we use on finite state machine design.

If we consider clock as a power supply to the circuit then output will not change irrespective of the bit dooropen and start. As no specifications is given for clock, we can consider it as rising edge clock pulse which starts process when clock will be 1 and when clock =0 then no operation will be performed.

Here one thing we must note that what is the use of next wash bit. When door will be open then wash will be zero and therefore next wash will be zero. But when door is closed and and start bit is 1 then wash will be 1 and hence next wash will be 1 or 0, why you will use of your washing is done, hence it will be (X) i.e. 0 or 1 depends on user.

If we consider clock as input signal then state table as given below,

Now consider clock as external input then state table is

Arrow represent when start is 0 and door is closed then wash will be previous state. When door is open, then wash must be 0 irrespective of start bit.

Note- please find me in comment box if you find any query.