Semiconductor Memory and Programmable Logic Devices

Posted on Jan 2, 2026 in Electronic Systems Engineering

A) Semiconductor Memory: RAM, ROM, EPROM, and EEPROM

(7 Marks)

1. RAM (Random Access Memory)

Definition: RAM is a type of volatile memory that stores data and instructions currently in use by the processor. It allows the Central Processing Unit (CPU) to access data in any order, hence the term “Random Access.”

Characteristics:

• Volatile: Data is lost when the power is turned off.
• Fast: It provides quick data access and is essential for efficient processing.
• Read/Write: It supports both the reading and writing of data.

Types:

• DRAM (Dynamic RAM): It needs to be refreshed periodically; it is slower but more common.
• SRAM (Static RAM): It is faster and more reliable, and it does not need refreshing.

Uses: It stores running programs, operating systems, and temporary data. It is essential for smooth multitasking.

2. ROM (Read-Only Memory)

Definition: ROM is a non-volatile memory used to store permanent instructions required for the system to start (boot) and function properly.

Characteristics:

• Non-volatile: It retains data even when the system is powered off.
• Read-only: Typically, data is written once and cannot be modified easily (except in certain types like EEPROM or EPROM).

Uses: It stores firmware, bootstrap code, and system startup instructions.

Types:

• PROM (Programmable ROM): It can be programmed once after manufacture.
• EPROM (Erasable Programmable ROM): It can be erased by ultraviolet (UV) light and reprogrammed.
• EEPROM (Electrically Erasable Programmable ROM): It can be erased and reprogrammed electrically.

3. EPROM (Erasable Programmable Read-Only Memory)

Definition: EPROM is a type of ROM that can be erased using ultraviolet (UV) light and reprogrammed.

Characteristics:

• Non-volatile: Data remains stored without power.
• Erasable: It requires exposure to UV light to erase, making it reusable.
• Read-only: Data is typically written once, but it can be reprogrammed after erasure.

Uses: Firmware storage where updates are infrequent, such as in embedded systems or hardware initialization code.

4. EEPROM (Electrically Erasable Programmable Read-Only Memory)

Definition: EEPROM is a non-volatile memory that allows data to be erased and rewritten electrically, without the need for UV light.

Characteristics:

• Non-volatile: Data is retained even when the system is powered off.
• Electrically Erasable: It allows for byte-level erasure and reprogramming.
• Slower than RAM: While it offers flexibility, it is not as fast as RAM.

Uses: Commonly used to store small, critical data such as device configuration settings, user preferences, or calibration data in embedded systems, microcontrollers, and BIOS.

A) Programmable Logic Devices (PLDs)

(Suitable for 7–8 marks)

Diagram (Generic PLD Structure)

              Inputs
                │
     ┌──────────┴──────────┐
     │   Programmable      │
     │       AND Array      │
     └──────────┬──────────┘
                │  (Product Terms)
     ┌──────────┴──────────┐
     │   Programmable /     │
     │   Fixed OR Array     │
     └──────────┬──────────┘
                │
            Output Logic
        (Buffers / FFs / I/O)
                │
              Outputs

Explanation

Programmable Logic Devices (PLDs) are digital Integrated Circuits (ICs) in which the logic functions can be configured by the user after manufacturing. Instead of designing custom hardware, PLDs allow the implementation of logic functions by programming internal connections.

Types of PLDs

• PROM (Programmable Read-Only Memory): Contains a fixed AND array and a programmable OR array. Used mainly for combinational logic.
• PAL (Programmable Array Logic): Has a programmable AND array and a fixed OR array. It features a faster and simpler structure.
• PLA (Programmable Logic Array): Has a programmable AND and a programmable OR array. It is more flexible than PAL.
• CPLD (Complex PLD): A combination of multiple PAL/PLA blocks. Suitable for moderate-complexity digital systems.
• FPGA (Field Programmable Gate Array): Contains configurable logic blocks (CLBs), programmable interconnects, and I/O blocks. Used for very complex digital applications.

Working Principle

• Inputs feed into a programmable AND matrix to generate product terms.
• Product terms pass into an OR matrix to form required logic expressions.
• Outputs may include flip-flops, enabling sequential circuits.
• Programming technologies include EPROM, EEPROM, Flash, or SRAM, depending on the device type.

Advantages

• Reduces circuit complexity and PCB space.
• Reprogrammable and reconfigurable.
• Lower cost and faster prototyping.
• High reliability due to fewer external connections.

B) Comparison of PAL and PLA Architectures

Difference Between PAL and PLA

(Presented in tabular form for full marks)

Short Note on Semiconductor Memories (7 Marks)

Semiconductor memories are electronic memory devices made using semiconductor technology such as silicon. They store digital information in binary form (0s and 1s) and are essential components of computers, microprocessors, microcontrollers, and digital systems. They offer high speed, low power consumption, small size, and high reliability, making them the most widely used memory devices today.

1. Primary Classification

• Volatile Memory: Loses data when the power is switched off. Examples: RAM (SRAM, DRAM).
• Non-Volatile Memory: Retains data even when power is removed. Examples: ROM, PROM, EPROM, EEPROM, Flash.

2. RAM (Random Access Memory)

Used as read/write memory for temporary data storage.

• (a) SRAM – Static RAM: Uses flip-flops for storage. It is faster, more expensive, and has low density. Used for cache memory.
• (b) DRAM – Dynamic RAM: Uses capacitors to store charge. It has high density, is cheaper, and requires periodic refreshing. Used for main memory.

3. ROM (Read-Only Memory)

Used for permanent data storage such as firmware.

• Mask ROM: Programmed during manufacturing.
• PROM: User-programmable once.
• EPROM: Erasable using UV light; reprogrammable.
• EEPROM: Electrically erasable; byte-wise erasure.
• Flash Memory: Block-wise erasure; used in pen drives and SSDs.

4. Advantages of Semiconductor Memories

• Very high speed of operation.
• Low power consumption.
• Compact size and highly integrated.
• Reliable and easy to interface with microprocessors.

5. Applications

• Computers and servers.
• Embedded systems and microcontrollers.
• Mobile devices and IoT.
• Storage devices like SSDs and USB drives.

Sample and Hold Circuit Analysis

Explanation

A Sample and Hold Circuit is used to sample an analog input signal at a specific time and “hold” that value constant for a period. This is especially useful in systems like analog-to-digital conversion (ADC) where the input signal needs to be sampled at regular intervals.

• Sample: When the switch is closed, the input voltage is passed onto the capacitor.
• Hold: When the switch is open, the capacitor retains the last sampled voltage.

The operation of the circuit can be divided into two phases:

1. Sampling Phase: The switch is closed, and the capacitor charges to the input voltage.
2. Hold Phase: The switch is open, and the voltage across the capacitor remains constant (holds the sampled value).

Diagram

Here is a basic diagram for a Sample and Hold circuit:

    +--------+
    |        |
----|  Input |----  Switch ---->|   |      |
    |        |      (S)         | C |----> Output (Held Voltage)
    +--------+                  |   |
----                            -----

Where:

• S is the switch.
• C is the capacitor.
• The output holds the voltage across the capacitor during the hold phase.

Key Points

• Used in Analog-to-Digital Converters (ADCs) to stabilize the input signal.
• The capacitor ideally holds the input voltage without significant decay during the hold period.

B) R-2R Ladder Digital-to-Analog Converter

Explanation

An R-2R Ladder DAC (Digital-to-Analog Converter) uses a resistor ladder network to convert a binary number into an analog output voltage. It is a simple and cost-effective circuit used in many digital applications.

The key feature of the R-2R ladder is its use of only two resistor values: R and 2R.

• R is the standard resistor.
• 2R is twice the value of R.

The digital input is applied to switches connected to the resistor network, where each switch corresponds to a binary bit of the input.

Working

Each bit of the digital input controls a switch that connects either a reference voltage or ground to the corresponding node in the resistor ladder. The binary-weighted voltages are summed at the output, producing a corresponding analog output voltage that represents the input digital value.

Diagram

Here is the basic diagram of an R-2R ladder DAC:

           Vref
            |
           [R] 
            |
           [S1]---- Output
            |
           [2R] 
            |
           [S2] 
            |
           [R]
            |
           [S3]
            .
            .
            .
            |
           GND

Where:

• Vref is the reference voltage.
• R and 2R resistors create the ladder.
• S1, S2, S3… are the switches controlled by the binary input.
• The output is the analog voltage resulting from the weighted sum of the reference voltage across the resistors.

Key Points

• It is binary-weighted, with each resistor node contributing a fraction of the reference voltage.
• The output voltage is a direct function of the binary input.