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Hardware Design Interview Preparation Guide
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Learn Computer Hardware Design by our research based Computer Hardware Design Interview Questions and Answers. Its easy way to learn hardware, put your comments suggestions, more questions and answers to us related to the computer hardware designing.

## 20 Hardware Design Questions and Answers:

### 1 :: Draw a Transmission Gate-based D-Latch?

The Transmission-Gate's input is connected to the D_LATCH data input (D), the control input to the Transmission-Gate is connected to the D_LATCH enable input (EN) and the Transmission-Gate output is the D_LATCH output (Q)

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### 2 :: Suppose you have a combinational circuit between two registers driven by a clock. What will you do if the delay of the combinational circuit is greater than your clock signal?

Use the concept of register-retiming.

divide the total combinatorial delay in two segments such that individually the delay is less the clock period.

this can be done by inserting a flip-flop in the combinational path.

e.g,

clock period --- 5 ns

total cominational delay ---- 7

then divide the 7ns path in two path of 4 or 3 (best results are obtained if delays are same for both path i.e 3.5ns) by inserting a flip-flop in between.

divide the total combinatorial delay in two segments such that individually the delay is less the clock period.

this can be done by inserting a flip-flop in the combinational path.

e.g,

clock period --- 5 ns

total cominational delay ---- 7

then divide the 7ns path in two path of 4 or 3 (best results are obtained if delays are same for both path i.e 3.5ns) by inserting a flip-flop in between.

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### 3 :: How do you detect a sequence of "1101" arriving serially from a signal line?

Sequence detector : A sequence detector gives an output of 1 on detecting the given sequence else the output is zero.

Ex : if the given sequence to be detected is 111

and input stream is 1 1 0 1 1 1 0 0 1 0 1 1 1 1 1

the output should be 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1.

Soln:

One of the different possible ways to detect a sequence is using a Mealy type FSM.

Using the following table the State machine can be designed.

since the number of bits in the sequence 1101 is 4 we have 4 states

------------------------------------------------------

|PS | Seq detected by the state | NS/output |

| | |---------------|

| | | X=0 | X=1 |

|-----------------------------------------------------

| S1 | - | S1/0 | S2/0 |

|----------------------------------------------------|

| S2 | 1 | S1/0 | S3/0 |

|----------------------------------------------------|

| S3 | 11 | S4/0 | S3/0 |

|----------------------------------------------------|

| S4 | 110 | S1/0 | S2/1 |

|----------------------------------------------------|

when in state S4 (PS),and input(X) from the sequence is 1,the sequence "1101" has been detected once and (to find the next state select the longest "seq identified by a state" column that matches part of the sequence 1101--ie.,1 or 01 or 101 ....)the NS is S2 since the sequence detected by the state S2 is 1(in 1101- 01 or 101 ,etc are not present in the seq identified by the state column ,)

Ex : if the given sequence to be detected is 111

and input stream is 1 1 0 1 1 1 0 0 1 0 1 1 1 1 1

the output should be 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1.

Soln:

One of the different possible ways to detect a sequence is using a Mealy type FSM.

Using the following table the State machine can be designed.

since the number of bits in the sequence 1101 is 4 we have 4 states

------------------------------------------------------

|PS | Seq detected by the state | NS/output |

| | |---------------|

| | | X=0 | X=1 |

|-----------------------------------------------------

| S1 | - | S1/0 | S2/0 |

|----------------------------------------------------|

| S2 | 1 | S1/0 | S3/0 |

|----------------------------------------------------|

| S3 | 11 | S4/0 | S3/0 |

|----------------------------------------------------|

| S4 | 110 | S1/0 | S2/1 |

|----------------------------------------------------|

when in state S4 (PS),and input(X) from the sequence is 1,the sequence "1101" has been detected once and (to find the next state select the longest "seq identified by a state" column that matches part of the sequence 1101--ie.,1 or 01 or 101 ....)the NS is S2 since the sequence detected by the state S2 is 1(in 1101- 01 or 101 ,etc are not present in the seq identified by the state column ,)

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### 4 :: Give the truth table for a Half Adder. Give a gate level implementation of the same.

TRUTH TABLE FOR HALF ADDER:

A B SUM CARRY

0 0 0 0

0 1 1 0

1 0 1 0

1 1 0 1

IMPLEMENTATION:

For SUM, The two inputs A and B are given to XOR gate.

For Carry, The two inputs A and B are given to AND gate.

A B SUM CARRY

0 0 0 0

0 1 1 0

1 0 1 0

1 1 0 1

IMPLEMENTATION:

For SUM, The two inputs A and B are given to XOR gate.

For Carry, The two inputs A and B are given to AND gate.

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### 5 :: Design a divide-by-3 sequential circuit with 50% duty circle.

Take a smiths counter with 3 f/f's

that is to say with 6 states(2*3)

now double the i/p clock frequency to the counter

the o/p of the 3rd f/f is divide by 6 of the i/p with 50% duty cycle

so effectively u got divide by 3 freq with 50% duty cycle

that is to say with 6 states(2*3)

now double the i/p clock frequency to the counter

the o/p of the 3rd f/f is divide by 6 of the i/p with 50% duty cycle

so effectively u got divide by 3 freq with 50% duty cycle

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### 6 :: Give two ways of converting a two input NAND gate to an inverter.

One way is shorting the two inputs of the NAND gate and passing the input.

truth table:

A B output

1 1 0

0 0 1

The second way is passing the input to only one input(say A) of the NAND gate.Since the other input(say B) is floating, it is always logic one.

truth table:

A B output

1 1 0

0 1 1

truth table:

A B output

1 1 0

0 0 1

The second way is passing the input to only one input(say A) of the NAND gate.Since the other input(say B) is floating, it is always logic one.

truth table:

A B output

1 1 0

0 1 1

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### 7 :: How do you detect if two 8-bit signals are same?

XOR each bits of A with B (for eg A[0] xor B[0] ) and

so on. the o/p of 8 xor gates are then given as i/p to

an 8-i/p nor gate. if o/p is 1 then A=B.

so on. the o/p of 8 xor gates are then given as i/p to

an 8-i/p nor gate. if o/p is 1 then A=B.

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### 8 :: Give a circuit to divide frequency of clock cycle by two?

You can divide the frequency of a clock by just implementing T Flip flop.

Give clock as clock input and tie the T input to logic 1.

Give clock as clock input and tie the T input to logic 1.

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### 9 :: What are set up time & hold time constraints? What do they signify? Which one is critical for estimating maximum clock frequency of a circuit?

Suppose your flip-flop is positive edge triggered. time for which data should be stable prior to positive edge clock is called setup time constraint .

Time for which data should be stable after the positive edge of clock is called as hold time constraint.

if any of these constraints are violated then flip-flop will enter in meta stable state, in which we cannot determine the output of flip-flop.

there are two equation:

1. Tcq + Tcomb> Tskew + Thold

2. Tcq + Tcomb<Tskew +T - Tsetup

Tcq is time delay when data enters the flip-flop and data comes at output of flip flop.

Tcomb is the logic delay between two flip flop.

Tskew is the delay of clock to flip flop: suppose there are two flip flop ,if clock reaches first to source flip flop and then after some delay to destination flip flop ,it is positive skew and if vice versa then negative skew.

so if you take 2 eq you will see that setup time is the determining factor of clock's time period.

Time for which data should be stable after the positive edge of clock is called as hold time constraint.

if any of these constraints are violated then flip-flop will enter in meta stable state, in which we cannot determine the output of flip-flop.

there are two equation:

1. Tcq + Tcomb> Tskew + Thold

2. Tcq + Tcomb<Tskew +T - Tsetup

Tcq is time delay when data enters the flip-flop and data comes at output of flip flop.

Tcomb is the logic delay between two flip flop.

Tskew is the delay of clock to flip flop: suppose there are two flip flop ,if clock reaches first to source flip flop and then after some delay to destination flip flop ,it is positive skew and if vice versa then negative skew.

so if you take 2 eq you will see that setup time is the determining factor of clock's time period.

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### 10 :: What are the different Adder circuits you studied?

Adders are generally of five types:

1) Ripple Carry Adder:

The Ripple carry adder(RCA) consists of a building block named Half

Adder(HA) which is cascaded to form a Full Adder(FA). These building

blocks HAs and FAs are also the building blocks of all types of

adders.The n full adders are cascaded to form n bit RCA.

The full adder has three input pins(input Ai,input Bi,carryin Ci) and

two output pins(Sum and Ci+1).Its equations are:

Sum=Ai^Bi^Ci

Ci+1=Ai.Bi+Bi.Ci+Ai.Ci

2)Carry Lookahead Adder:

The Carry Lookahead Adder(CLA) reduces the delay as that in RCA. Let

Gi=Ai.Bi, and Pi=Ai^Bi, then Ci+1=Gi+Pi.Ci.

The expressions for Sum and Ci+1 is then defined completely in terms of

input pins rather wait for input carry to appear.

3)Carry Select Adder:

The carry select adder uses duplicate modules for each combination of

input carry(i.e. 1 and 0).The multiplexers then select the appropriate

sum and carry output according to the carry output of the preceding

stages.

1) Ripple Carry Adder:

The Ripple carry adder(RCA) consists of a building block named Half

Adder(HA) which is cascaded to form a Full Adder(FA). These building

blocks HAs and FAs are also the building blocks of all types of

adders.The n full adders are cascaded to form n bit RCA.

The full adder has three input pins(input Ai,input Bi,carryin Ci) and

two output pins(Sum and Ci+1).Its equations are:

Sum=Ai^Bi^Ci

Ci+1=Ai.Bi+Bi.Ci+Ai.Ci

2)Carry Lookahead Adder:

The Carry Lookahead Adder(CLA) reduces the delay as that in RCA. Let

Gi=Ai.Bi, and Pi=Ai^Bi, then Ci+1=Gi+Pi.Ci.

The expressions for Sum and Ci+1 is then defined completely in terms of

input pins rather wait for input carry to appear.

3)Carry Select Adder:

The carry select adder uses duplicate modules for each combination of

input carry(i.e. 1 and 0).The multiplexers then select the appropriate

sum and carry output according to the carry output of the preceding

stages.

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