Write short notes on: (a) PLA (b) FLASH RAM (c) Draw and explain monostable multivibrator.

(a) PLA:

PLA (Programmable Logic Array) is a type of programmable logic device (PLD) used to implement combinatorial logic functions. It consists of an array of programmable AND gates and OR gates that can be interconnected to form a variety of logic functions. PLAs are typically used for applications where the logic function is complex or changes frequently.

The basic structure of a PLA consists of an AND array, an OR array, and an input/output (I/O) array. The AND array is responsible for generating the product terms for the logic function, while the OR array is responsible for generating the sum terms. The I/O array connects the inputs and outputs of the PLA to the external world.

PLAs are programmed by specifying the connections between the AND gates, OR gates, and I/O pins. This can be done using a variety of methods, including:

• Mask programming: The PLA is programmed by physically altering the metallization layers of the chip. This is a permanent programming method that cannot be changed once the chip has been manufactured.
• Fuse programming: The PLA is programmed by blowing fuses to create the desired connections. This is a semi-permanent programming method that can be changed if necessary.
• EEPROM programming: The PLA is programmed by storing the programming data in an EEPROM. This is a non-volatile programming method that can be changed multiple times.

PLAs offer a number of advantages over other types of PLDs, including:

• High speed: PLAs are typically faster than other types of PLDs because they use hardwired connections instead of programmable interconnects.
• Low power consumption: PLAs consume less power than other types of PLDs because they do not require any external components.
• Compact size: PLAs are typically smaller than other types of PLDs because they do not require any external components.

(b) FLASH RAM:

FLASH RAM (Flash Random Access Memory) is a type of non-volatile memory that can be electrically erased and reprogrammed. It is widely used in a variety of electronic devices, including computers, smartphones, and digital cameras.

FLASH RAM is based on a floating-gate transistor. A floating-gate transistor is a MOSFET with an extra gate electrode that is not connected to any other part of the circuit. The charge stored on the floating gate determines the threshold voltage of the transistor, which in turn determines whether the transistor is ON or OFF.

To write data to a FLASH RAM cell, a high voltage is applied to the floating gate, which causes electrons to tunnel through the oxide layer and become trapped on the floating gate. This increases the threshold voltage of the transistor, causing it to turn OFF. To erase data from a FLASH RAM cell, a high voltage is applied to the drain of the transistor, which causes the electrons on the floating gate to be discharged. This decreases the threshold voltage of the transistor, causing it to turn ON.

FLASH RAM offers a number of advantages over other types of non-volatile memory, including:

• High speed: FLASH RAM is much faster than other types of non-volatile memory, such as EEPROMs and ROMs.
• Low power consumption: FLASH RAM consumes less power than other types of non-volatile memory.
• High endurance: FLASH RAM can be erased and reprogrammed thousands of times without degrading the performance of the memory cells.
• Scalability: FLASH RAM can be scaled to very high densities, making it suitable for a wide range of applications.

(c) Monostable Multivibrator:

A monostable multivibrator is a type of electronic circuit that generates a single output pulse of a predetermined duration when triggered by an input pulse. It is also known as a one-shot multivibrator.

The basic circuit of a monostable multivibrator consists of two transistors, two resistors, and a capacitor. The transistors are connected in a cross-coupled configuration, with the collector of each transistor connected to the base of the other transistor. The resistors are connected between the bases of the transistors and the positive supply rail, and the capacitor is connected between the collector of one transistor and ground.

The operation of a monostable multivibrator can be divided into two phases:

• Triggering phase: When an input pulse is applied to the base of one of the transistors, it causes the transistor to turn ON. This causes the collector voltage of the transistor to decrease, which in turn causes the base voltage of the other transistor to increase. This causes the other transistor to turn OFF, which causes the collector voltage of the transistor to increase.
• Recovery phase: After the input pulse is removed, the capacitor begins to discharge through the resistor connected to the base of the transistor that is OFF. This causes the base voltage of the transistor to decrease, which in turn causes the transistor to turn ON. This causes the collector voltage of the transistor to decrease, which in turn causes the base voltage of the other transistor to increase. This causes the other transistor to turn OFF, which causes the collector voltage of the transistor to increase.

The duration of the output pulse is determined by the values of the resistor and capacitor. The output pulse width can be calculated using the following formula:

$$t = RC \ln(2)$$

where:

• t is the output pulse width
• R is the resistance of the resistor
• C is the capacitance of the capacitor

Monostable multivibrators are used in a variety of applications, including:

• Timers: Monostable multivibrators can be used to generate precise timing pulses.
• Pulse generators: Monostable multivibrators can be used to generate a train of pulses.
• Flip-flops: Monostable multivibrators can be used to implement flip-flops.
• Counters: Monostable multivibrators can be used to implement counters.