JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV SYSTEM DESIGN USING FPGAS Previous Question Paper (December - 2020 - R16)

A three-state buffer (or tri-state buffer) is a digital logic gate that provides three output states: logic 0 (low), logic 1 (high), and a high-impedance state (Z). The high-impedance state effectively disconnects the output from the circuit, allowing multiple devices to share the same bus line without causing electrical contention.

The buffer consists of a data input, an output, and an enable control signal. When the enable signal is active (e.g., high), the output follows the input. When the enable signal is inactive (e.g., low), the output enters the high-impedance state, behaving as an open circuit.

In FPGA design, tri-state buffers are essential for implementing bidirectional buses. For example, in a system where a microprocessor communicates with memory and I/O peripherals over a shared data bus, tri-state buffers ensure that only one device drives the bus at any given time, preventing short circuits between conflicting logic levels.

An HDL (Hardware Description Language) is a specialized programming language used to describe the structure, behavior, and timing of digital electronic circuits. Unlike software programming languages, HDLs are designed to model concurrent hardware operations and physical connectivity.

The comparison between HDLs and standard high-level languages (like C or Java) is as follows:

• Concurrency: HDLs are inherently parallel; all statements execute simultaneously to model hardware components. High-level languages are typically sequential, executing instructions one after another.
• Execution Model: HDLs describe physical hardware synthesis and simulation of logic gates and flip-flops. High-level languages are compiled into machine instructions for execution on a CPU.
• Timing: HDLs include explicit constructs for modeling propagation delays and clock cycles critical to hardware timing. High-level languages focus on algorithmic logic and data processing without inherent physical timing constraints.
• Data Types: HDLs utilize bit-level data types (e.g., std_logic, bit vectors) to represent physical wires and buses, whereas high-level languages use abstract types like integers, floats, and objects.