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Name: 1
CSE 240 Autumn 2005 DUE: Fri. 23 September 2005
Intro. to Computer Architecture Homework 2
Write your name at the top of each page, and write your answers
on these pages. Additional pages may be attached
(with staple) if necessary. Please ensure that your answers are
legible. Please show your work. Due at the beginning
of class. Total points: 77.
1. [10 Points] Transistors. In this problem you will determine
for all possible inputs, A and B, whether or not each
of the transistors in the circuit, below, conduct or not.
A
B
Z
3
1
2
6
4
5
(a) For all inputs A and B indicate in the table below whether
each transistor (labeled 1-6) conducts (i.e., act
like a piece of wire, denoted 1) or does not conduct (i.e., acts
like an open circuit, denoted 0). Also
indicate the value (0 or 1) of the output Z.
A B 1 2 3 4 5 6 Z
0 0
0 1
1 0
1 1
(b) Give the logical expression that describes Z in terms of A
and B. You are better off basing your answer on
the circuit instead of the truth table.
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22. [5 Points] Transistors. The transistor-level circuit given
below has a problem: with certain inputs it catches on
fire!! Why is this? Be precise in describing when and why the
problem arises. Hint: You may find it useful to
determine whether each transistor in the circuit conducts and
the value of the output Z for each possible input.
A
Z
C
C
B
B
A
Cn
Bn
An
Cp
Bp
Ap
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Name: 3
3. [8 Points] Transistors Build a transistor-level circuit
implementing a 3-input AND gate. The output (Z) of the
circuit should be true only when all inputs (A, B, and C) are
true. Be sure to label A, B, C, and Z in your circuit.
Hint: If you are using more than 8 transistors, youre on the
wrong track.
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44. [12 Points] Combinational Logic Circuits. NOR is logically
complete (i.e., any logic function can be built
using only NOR gates). Using only NOR gates, construct
gate-level circuits that compute each of the following.
You might find it helpful to use DeMorgans Law.
(a) NOT
(b) OR
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Name: 5
(c) AND
(d) NAND
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65. [20 Points] Combinational Logic Circuits. The exclusive-or
function (XOR) is defined to be true when exactly
one of its two inputs is true.
(a) Confirm that Z = (A OR B) AND NOT (A AND B) computes XOR by
completing the following truthtable.
A B A OR B A AND B NOT (A AND B) Z = (A OR B) AND NOT (A AND
B)
0 0
0 1
1 0
1 1
(b) Construct the gate-level logic circuit for Z (above) using
only AND, OR, and NAND gates (no other gates
may be used). Be sure to label A, B, and Z.
(c) How many MOS transistors does your circuit require if AND,
OR and NAND gates are implemented as
given in Figures 3.6 and 3.7 of the textbook (pages 56 and
57).
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Name: 7
(d) Now confirm that Z = NOT ( NOT (A OR B) OR (A AND B)) also
represents XOR by completing thefollowing truth table. (If you
like, you can also symbolically show Z = Z via DeMorgans Law.)
A B A OR B NOT (A OR B) A AND B Z = NOT ( NOT (A OR B) OR (A AND
B))
0 0
0 1
1 0
1 1
(e) Construct the gate-level logic circuit for Z (above) using
only AND and NOR gates.
(f) How many MOS transistors does your circuit require if AND
and NOR gates are implemented as given in
Figures 3.5 and 3.7 of the textbook (pages 55 and 57).
