Lecture 7

Question 1

Two-level logic

Answer 1

Consists of literals connected to a level of AND gates connected to a level of OR gates

May use less HW

Question 2

Contention

Answer 2

When a node is being driven to both 0 and 1 at the same time (X)

Actual voltage somewhere between 0 and Vdd

Question 3

X: uses

Answer 3

By circuit simulators to indicate an uninitialized value (if you forget to specify the value of an input)

“Don’t care” values in truth tables (value of variable unimportant)
-in circuit (node has unknown/illegal value)

Question 4

Z

Answer 4

Circuit node being driven neither HIGH nor LOW

Question 5

Common ways to produce a floating node

Answer 5

Forgetting to connect a voltage to a circuit input

Falsely assuming that an unconnected input is the same as an input with the value of 0 (causes circuit to behave erratically as floating input changes from 0 to 1 randomly)

Touching the circuit may be enough to trigger the change by means of static electricity

Question 6

Tristate buffer output states

Answer 6

HIGH (1), LOW (0), floating (Z)

Active high (E=1)

• When E TRUE - transferred input value to output
• When E FALSE - output allowed to float (Z)

Question 7

Tristate buffer: active low

Answer 7

E=0

Y = Z

Bubble on input wire

Question 8

Tristate buffer uses

Answer 8

May facilitate communication between a microprocessor, video controller, and an Ethernet controller as well as memory