Logic Design 2 - Clarke University

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2 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Transistors:

Are used to implement NOT gates from which we can construct NAND and NOR gates.
Since NAND and NOR gates are Universal Gates( see P.42 R.B), then by using Transistors
we can build all other gates(P. 43

T.B) and thus all circuits( such as full adders, full
subtractors , etc.).


In Transistors
we connect a positive voltage ( such as +5) to one end and 0 Voltage (Ground) to the
other end. By having such a gradient in voltage electron
s will flow (
emit
) from the ground to be
collected

at the other end(+5). To be able to treat a Transistor as a NOT gate we obviously must
impose some kind of switch mechanism which switch the Transistor to an On/Off state.

To be able to get a switch oper
ation we connect another input (beside the Ground input) at the Base
of the Transistor. When Base is +( > 0) , it will pull the zero voltage of the Ground up to the
Collector. The V
0

Output will thus record a 0 ( the ground voltage). Otherwise, when the Ba
se is 0
there will be no flow from Ground to Collector. Thus the V
0

output will read the positive voltage at
the near end(+5).

Q: What happen if:


(1) the Base is negative?

(2)

when both ends are at 0 Voltage.

(3)


When both ends are at Positive voltage.

Q: Is

Transistors a One State, Two States or Three States Devices?


Q: What would you suggest to make the NOR gate connection
behave
as
a
n


OR Gate.


Q. What would you suggest to make the NAND gate connection
behave
s as



a
n AND G
ate.


Q. Design an XOR Gate using Transistors




Hint: X


Y = X . Y + X . Y,




we can build an AND Gate as:











X








Prove by Constructing a Truth Table




X .Y

Y

X

Q. Use Transistors only to solve





X + Y = X.X. Y.Y




M
ake
sure that you understand that the Truth table for the above is:



X


Y


X+Y


X.X



Y.Y


X.X. Y.Y


0

0

0

1

0

0

0

1

1

1

1

1

1

0

1

0

0

1

1

1

1

0

1

1







Q: Use Transistors to solve



X


Y =† ⹙ ‫† ‮⁙



Sequential Circuits:


Used to C
ontrol( using Tri
-
state Devises) and Store Data to be used in later time( Flip
Flop Devises).



I
nput


output


control




Tri
-
State Devise


Tri
-
State Devises

are used to produce outputs only when the associated control signal
is high (1). Otherwise, no output will be generated.

By connect
ing Tri
-
State devices to other multi
-
devices via a Bus Structure, we can
activate or deactivate all connected devices using one
-
control signals.

Such devices act as an IF Statement where we either produce an output or not.




D
-

Type Flip
-
Flop Devices





Q


q





D

CK









Used to Store( and thus delay) data in a giving processing operation. In Edge
-
Triggered Type, outputs represent data of a previous step in the FF. Thus whenever
CK is changed from 0 to 1, two activities take place:

(1)

the st
ored data is flipped( 0 to 1, or 1 to 0).

(2)

The output generate data before the flipping.


These devises are thus used to delay a given input data one Clock until other data are
ready.