8085 Microprocessor Architecture

Course: B.Sc-CS-II Subject: Introduction to

Microprocessor Unit-2

1

1) Combinational

2) Sequential• Combinational logic circuits (circuits without a

memory):Combinational switching networks whose outputs

depend only on the current inputs.• Sequential logic circuits (circuits with memory): In this kind of network, the outputs depend on the

current inputs and the previous inputs. These networks employ storage elements and logic gates.

LOGIC CIRCUITS

COMBINATIONAL CIRCUITS[1]

• Most important standard combinational circuits are:

• Adders

• Subtractors

• Comparators

• Decoders

• Encoders

• Multiplexers

Available in IC’s as MSI and used asstandard cells in complex VLSI (ASIC)

ANALYSIS OF COMBINATIONAL LOGIC[1]

ANALYSIS OF COMBINATIONAL LOGIC

ANALYSIS OF COMBINATIONAL LOGIC

DESIGN OF COMBINATIONAL LOGIC

1. From the specifications of the circuit, determine the number of inputs and outputs

2. Derive the truth table that defines the relationship between the input and the output.

3. Obtain the simplified Boolean function using x-variable K-Map.

4. Draw the logic diagram and verify the correctness of the design.

DESIGN OF COMBINATIONAL LOGIC[2]

• Example: Design a combinational circuit with three inputs and one output. The output is a 1 when the binary value is less than three.

• The output is 0 otherwise.

BINARY ADDER – Half Adder[2]

Implementation of Half Adder

BINARY ADDER - Full Adder[2]

Full Adder in SOP[2]

Implementation Full Adder with two half Adders[2]

4-BIT FULL ADDER[2]

4 bit Adder

Magnitude Comparator• A magnitude comparator is a combinational circuit that compares

two numbers, A and B, and then determines their relative magnitudes.

A > B A = B A < B• Algorithm Consider two numbers, A and B, with four digits each: A=A3 A2 A1 A0

B=B3 B2 B1 B0

• xi=1 if A=B=0 or A=B=1

• xi=AiBi+ Ai’Bi’ for i=0,1,2,3 XNOR

• For equality to exist, all xi variables must be equal to 1:

• (A=B)=X3 X2 X1 X0

AND Operation

Magnitude Comparator

• To determine if A is greater than or less than B, we inspect the relative magnitudes of significant digits.

• If the two digits are equal, we compare the next lower significant pair of digits. The comparison continues until a pair of unequal digits is reached.

• The sequential comparison can be expressed by:

DECODERS

• A decoder is a combinational circuit that converts binary information

• from n input lines to an 2nunique output lines.• Some Applications:

a) Microprocessor memory system: selecting different banks of memory.

b) Microprocessor I/O: Selecting different devices.

c) Memory: Decoding memory addresses (e.g. in ROM).

d) Decoding the binary input to activate the LED segments so that the decimal number can be displayed.

3-to-8-line DECODER[3]

• If the input corresponds to minterm mi then the decoder ouput i will be the single asserted output.

3-to-8-line DECODER[3]

2-to-4-line DECODER with Enable[3]

• The decoder is enabled when E = 0. The output whose value = 0 represents the minterm is selected by inputs A and B.

• The decoder is inactive when E=1 D0………D3 =1

• A Decoder with enable input is called a decoder/demultiplexer. Demultiplexer receives information from a single line and directs it to the output lines.

A 4 x 16 DECODER[3]• When w = 0, the top decoder is enabled and the bottom is disabled.• Top decoder generates 8 minterms 0000 to 0111, while the bottom

decoder outputs are 0’s.• When w = 1, the top decoder is disabled and the bottom is enabled.• Bottom decoder generates 8 minterms 1000 to 1111, while the top

decoder outputs are 0’s.

Full-Adder using Decoder[4]

MULTIPLEXERS/DATA SELECTORS[4]

• A multiplexer is a combinational circuit that selects one of many input lines (2n ) and steers it to its single output line. There are (2n ) and n selection lines whose bit combinations determine which input is selected.

4-to-1LINE MULTIPLEXER DESIGN[4]

• In general, a 2n –to–1- line multiplexer is constructed from an n–to 2n decoder by adding to 2n it lines, one to each AND gate.

References

1. Computer Organization and Architecture, Designing for performance by William Stallings, Prentice Hall of India.

2. Modern Computer Architecture, by Morris Mano, Prentice Hall of India.

3. Computer Architecture and Organization by John P. Hayes, McGraw Hill Publishing Company.

4. Computer Organization by V. Carl Hamacher, Zvonko G. Vranesic, Safwat G. Zaky, McGraw Hill Publishing Company.