ROMs, PLDs, PLAs, state reduction,

 

Overview of Digital Logic Design

Characteristics of Logic Gate Families

Logic gate families are groups of electronic devices that use different technologies for implementation. Some of the most common logic families include:

TTL (Transistor-Transistor Logic)

• Characteristics: High speed, moderate power consumption, noise immunity.
• Advantages: Widely used, reliable.
• Disadvantages: Consumes more power compared to CMOS.

CMOS (Complementary Metal-Oxide-Semiconductor)

• Characteristics: Low power consumption, high noise immunity, slow switching speed compared to TTL.
• Advantages: Energy-efficient, suitable for battery-operated devices.
• Disadvantages: More susceptible to damage from static electricity.

ECL (Emitter-Coupled Logic)

• Characteristics: Very high-speed, low noise margin, high power consumption.
• Advantages: Fastest logic family.
• Disadvantages: High power dissipation makes it less suitable for dense circuits.

BiCMOS (Bipolar CMOS)

• Characteristics: Combines the speed of TTL and low power consumption of CMOS.
• Advantages: Optimized for analog and digital integration.
• Disadvantages: Complex manufacturing process.

Read-Only Memory (ROM), Programmable Logic Devices (PLDs), and Programmable Logic Arrays (PLAs)

ROM (Read-Only Memory)

• Characteristics: Non-volatile storage; data is permanently programmed.
• Use: Fixed data storage like firmware, lookup tables, and microcode.

PLDs (Programmable Logic Devices)

• Types: PAL (Programmable Array Logic), GAL (Generic Array Logic).
• Characteristics: Customizable logic circuits; users can program them to perform specific logic functions.
• Use: Used in digital design for customizable and flexible logic implementation.

PLAs (Programmable Logic Arrays)

• Characteristics: Both AND and OR planes are programmable.
• Use: Greater flexibility than ROMs and PALs; used for implementing complex logic circuits and combinational logic.

State Reduction and Good State Variable Assignments

State Reduction

• Objective: Minimize the number of states in a state machine without altering the input-output relationship.
• Method: Combine states with identical future behavior for all inputs.
• Importance: Simplifies circuit design, reduces cost, and increases reliability.

State Variable Assignments

• Objective: Assign binary values to states in a way that simplifies the logic expressions for the next state and output.
• Importance: Proper state variable assignment can reduce the complexity of combinational logic circuits in sequential systems.

Algorithmic State Machine (ASM) Charts

An ASM chart is a graphical representation of a sequential circuit that combines state diagrams and flowcharts.

• State Boxes: Represent the states of the circuit.
• Decision Boxes: Represent the conditions that determine state transitions.
• Conditional Output Boxes: Indicate outputs that are conditional upon certain states.
• Use: Used in designing and documenting digital sequential circuits, particularly for complex state machines.

Asynchronous Circuits

Asynchronous circuits are circuits that do not rely on a global clock signal; they change states based on the occurrence of input changes.

• Characteristics: Faster than synchronous circuits since there is no clock synchronization delay; more difficult to design due to timing challenges and hazards.
• Use: Power-sensitive applications, high-performance systems, and systems where global clock distribution is impractical.