Page 185 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes
• 1949 – monostable as flip-flop: “... a flip-flop is a monostable multivibrator and the
ordinary multivibrator is an astable multivibrator.”
According to P. L. Lindley, a JPL engineer, the flip-flop types discussed below (RS, D, T, JK)
were first discussed in a 1954 UCLA course on computer design by Montgomery Phister, and
then appeared in his book Logical Design of Digital Computers. Lindley was at the time working
at Hughes Aircraft under Dr. Eldred Nelson, who had coined the term JK for a flip-flop which
changed states when both inputs were on. The other names were coined by Phister. They differ
slightly from some of the definitions given below. Lindley explains that he heard the story of
the JK flip-flop from Dr. Eldred Nelson, who is responsible for coining the term while working
at Hughes Aircraft. Flip-flops in use at Hughes at the time were all of the type that came to
be known as J-K. In designing a logical system, Dr. Nelson assigned letters to flip-flop inputs
as follows: #1: A & B, #2: C & D, #3: E & F, #4: G & H, #5: J & K. Nelson used the notations
“j-input” and “k-input” in a patent application filed in 1953.
Questions:
1. What is importance of electronic flip-flop?

2. How many types of flip-flop?

Self Assessment

Multiple choice questions:
6. Sequential circuits with ....................... can cause the circuit to produce erroneous behaviour.
(a) unused states (b) states assignment
(c) state reduction (d) states hazard
7. The ..................... is constructed using all the states of the sequential circuit in question.

(a) state diagram (b) states assignment
(c) state reduction (d) states hazard
8. Combinational logic decodes the state into the ....................... signals.
(a) next-state (b) input
(c) clock (d) output

True or False:
9. The K-map is the same as the combinational circuits K-map.

(a) True (b) False
10. In particular, nearly all computers are not designed as clocked sequential systems.
(a) True (b) False

10.6 Summary

• Digital electronics is classified into combinational logic and sequential logic.
• A sequential circuit is specified by a time sequence of inputs, outputs, and internal states.
• Synchronization is achieved by a timing device called a clock pulse generator.
• A clock signal is a periodic square wave that indefinitely switches from 0 to 1 and from
1 to 0 at fixed intervals.

180 LOVELY PROFESSIONAL UNIVERSITY