Saturday, November 15, 2014

GATE Previous Questions on Memories(ROM, PLA and PAL) & CMOS Logic Gates with Solutions (1987 - Till Date)


1.       Two dimensional addressing of 256 X 8 bit ROM using 8 to 1 selectors requires _______ (how many?) NAND gates.
Answer:     2304

2.       The CMOS equivalent of the following nMOS gate (shown in figure) is ________ (Draw the circuit).

Answer:  (A + BC)'

3.       A bit stored in a FAMOS device can be erased by ___________.
  Answer:  UV Light

4.       In the figure, the Boolean expression for the output in terms of inputs A, B and C when the clock CK is high, is given by ____________

  Answer:  (A + B)C


1.       Choose the correct statement(s) from the following:
            a.       PROM contains a programmable AND array and a fixed OR array
            b.      PLA contains a fixed AND array and a programmable OR array
            c.       PROM contains a fixed AND array and a programmable OR array
            d.      PLA contains a programmable AND array and a programmable OR array

Answer:    C & D


               1.        A microprocessor has five address lines [A4 – A0] and eight data lines [D7 – D0]. An input  device A, an output device B, a ROM and a RAM are memory mapped to the microprocessor at the addresses as shown in figure. Devices A and B have four addressable registers each/ RAM has 8 bytes and ROM has 16 bytes.
                a.       Indicate the address lines to be connected to each device and memory.
                b.      Obtain the minimum sum of product expression for the chip select (CS) function of  each  device/memory.


1.       A PLA can be used
            a.       As a microprocessor
            b.      As a dynamic memory
            c.       To realize a sequential logic
            d.      To realize a combinational logic
Answer:   D

2.       A dynamic RAM consists of
            a.       6 transistors
            b.      2 transistors and 2 capacitors
            c.       1 transistor and 1 capacitor
            d.      2 capacitors only
Answer:  C

3.       A 2 µsec pulse can be stretched into a 10 msec pulse by using a _______ circuit.
Answer:  Mono Stable Multivibrator


1.       The minimum number of MOS transistors required to make a dynamic RAM cell is
            a.       1
            b.      2
            c.       3
            d.      4
Answer:   A

2.   A ROM is used to implement the Boolean functions given below.

            a.       What is the minimum size of the ROM required?
            b.      Determine the data in each location of the ROM


1.       A Dynamic RAM cell which holds 5 volts has to be refreshed every 20 ms, so that the stored voltage does not fall by more than 0.5 volts. If the cell has a constant discharge current of 0.1 pA, the storage capacitance of the cell is
            a.       4 x 10-5 Farads 
            b.      4 x 10-9 Farads
            c.       4 x 10-12 Farads
            d.      4 x 10-15 Farads
               Solution : 

2.       A memory system of size 26K bytes is required to be designed using memory chips which have 12 address lines and 4 data lines each. The number of such chips required to design the memory system is
            a.       2
            b.      4
            c.       8
            d.      13
               Solution : 

3.   It is desired to generate the following three Boolean functions using an OR array as shown in figure, where P1 to P5 are the product terms in one or more of the variables a, a’, b, b’, c and c’. 

      Write down the terms P1, P2, P3, P4 and P5.

               Solution :     


1.       Each cell of a Static Random Access Memory contains
            a.       6 MOS transistors
            b.      4 MOS transistors and 2 capacitors
            c.       2 MOS transistors and 4 capacitors
            d.      1 MOS transistor and 1 capacitor

Answer:  A
                  Solution :  

2.       For the NMOS logic gate shown, the logic function implemented is

Answer:  C
                  Solution : 

3.   Match the following :

Answer:  1-b, 2-d
                  Solution : 


1.   For the CMOS monostable multivibrator of given figure, R = 50 KΩ and C = 0.01 µF, VDD = 5 volts, and the CMOS NOR gates have a threshold voltage (VT) of 1.5 volts. Vin is a trigger pulse (τp << RC) as shown in the figure.
            a.       Plot VA1 and Vo1 as functions of time.
            b.      Write the equation for VR(t), for t > 0
            c.       Find the time period of the output pulse.

                 Solution :


1.       An 8085 microprocessor based system uses a 4K x8 bit RAM whose starting address is AA00H. The address of the last byte in this RAM is
            a.       0FFFH
            b.      1000H
            c.       B9FFH
            d.      BA00H
Answer:  C
           Solution :

2.       In the DRAM cell in the figure is the VT of the N-MOSFET is 1 volt. For the following three combinations of WL and BL voltages.
            a.       5V; 3V; 7V 
            b.      4V; 3V; 4V
            c.       5V; 5V; 5V
            d.      4V; 4V; 4V
Answer:  B
           Solution :


1.       The circuit in the figure is ahs two CMOS NOR gates. This circuit functions as a 
            a.       Flip-flop
            b.      Schmitt trigger
            c.       Monostable multivibrator
            d.      Astable multivibrator
Answer:  C
                 Solution :  


1.   In the circuit shown in the figure, ‘A’ is a parallel in, parallel out 4 bit shift register, which loads at the rising edge of the clock C. The input lines are connected to a 4 bit bus, W. Its output acts as the input to a 16 X 4 ROM, whose output is floating when the enable input E is 0.
        A partial table of the contents of the ROM is as follows.

  The clock to the register is shown, and the data on the W bus at time t1 is 0110. The data on the bus at  time t2 is
            a.       1111
            b.      1011
            c.       1000
            d.      0010

Answer:   C
                   Solution :


1.       What memory address range is NOT represented by chip #1 and chip #2 in the figure. A0 to A15 in this figure are the address lines and CS means chip select.
            a.       0100 – 02FF
            b.      1500 – 16FF
            c.       F900 – FAFF
            d.      F800 – F9FF
Answer:   D
                  Solution :


1.       There are four chips each of 1024 bytes connected to a 16 bit address bus as shown in the figure below. RAMs 1,2,3 and 4 respectively are mapped to addresses
  Answer:  D
                Solution :


1.       In the following circuit employing pass transistor logic, all NMOS transistors are identical with a threshold voltage of 1 volt. Ignoring the body effect, the output voltages at P, Q, and R are
Answer:  C
                Solution :

2.       If WL is the Word Line and BL is the Bit Line, an SRAM cell is shown is
                Answer:   B
                 Solution :

3.       The output (Y) of the circuit shown in the figure is

  Answer:  A
                Solution :

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