Digital Interview Questions - Part 2

Q: Design a transparent latch using 2:1 mux ?
A: Enable pin of latch is the select pin of Mux. 1th pin of Mux is connected is the input of Latch & oth pin is fed by output(feedback).

Q: Design a 4:1 mux using 2:1 mux
A: We need three 2:1 mux for our design . 4 input is given as the inputs of two 2:1 mux. Sel0 is connected with the sel pin of these mux. Output of these mux given to another(third) 2:1 mux & Sel1 is given as the sel pin of third 2:1 mux .

Q: What is metastability ?
A: Whenever a flipflop violate setup or hold time, flop enters to a state where the output is unpredictable . This state is known as metastable state. At the end of metastable state output will settle to 0 or 1.This process is known as metastability .

Q: Convert Nand gate to Not gate in two different ways
A:
1. Nand gate inputs are connected together and give as input of Not gate .
2. Make one input of Nand gate as high & other input as the input of Not gate.

Q:What is clock Skew ?
A: Clock skew is the phenomenon in synchronous circuit in which clock signal arrives at diferent component at different times . This may be because of interconnect wire delay or temperature variations or material imperfection or anything something like that.
There are two types of clock skew : negative clock skew and Positive clock skew,Positive skew occur when the register receive clock later than the expected time . Negative skew is just opposite to this,ie register receive clock earlier than the expected time.

Q: What is race condition ?
A: Race condition is nothing but the condition in which output of gate or flop or circuit reaches a unexpected state that can affect the other part of circuit,ie just like a glitch.
For example consider a And gate assume that same input A is fed to two inputs of gate in such a way that one one input of gate is A and other one is !A . There will be chance to produce a glitch when A changes from one state to another because of internal delay in not gate(!A).

Q: Which one is good Asynchronous reset or Synchronous reset ?
A: Its advisable to use Asynchronous reset because we can make circuit to a reset state before getting clock . Other wise there is a chance of propagation of unwanted value b4 reset occur,which is not advisable .

Q:What is the difference between Mealy and Moore state machine ??
A: Mealy and Moore are different way of designing FSM .
Mealy State machine will depends on both input and state but Moore machine depends only on State not the Input . Advantage of moore machine is that the output will always synchronous to clock ,in Mealy machine,if the input is not synchronous with the clock may results in glitches/unknown values .

Gated Clocks.

Its a very good idea to turn off clocks when it is not needed. This is known as clock gating .

Why we need Clock gating ??
1. To save power consumed by combination circuit whose value is changed in each clock .
2. To save power consumed by flipflop.

Automatic clock gating is support by modern EDA tools .

There are two type of clock gating available .They are
1. Using combination circuits :-simple but glitches may come .
2. FF based circuits : Glitches free .

Circuits are given above .

Clock Muxing

Now a days we are handling multiple clock domains in our design . So we are always encounter with the problem that the switching between two clocks with out glitches.

There are two different chances,one two clock domains are totaly asynchronous or another one is one clock is multiple of another clock . In these two cases there is a chance for producing glitch while the switching between two clocks.

Usally we will disable both clock before swithing after that we change select pin & then enable clock for clock muxing.It is the safer methord of clockmuxing .

Here figures shows differents clocks muxing techniques .This is from an article in EE times. For more details click here

Digital Interview Questions - Part 1

Q : If inverted op of D ff is connected to input of ff how the flop behave ??
A : It behave as T flipflop

Q : Design a circuit for dividing input frequency by 2 ?
A : please give input frequency as the clock of T flip flop & input of flop is 1. Output will be input
clock divide by 2 .

Q : What are the different type of adder implementation ?
A : Carry Ahead,And & Xor combinational circuit , Ripple carry .

Q: Give the Truth Table for a Half Adder ?? Give gate level Implementaion of it ??
A:
A B Y Carry
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1

Y = A xor B , Carry = A and B

Q : Design OR gate from 2:1 mux
A : assume that A & B are the input of OR gate . Then
Give A as sel pin of 2:1 mux , B to the 0th pin Mux & 1 to 1th pin of Mux

Q : Design a D FF from two latches ?
A : Connect two latches serialy , First latch enable pin positive level & second latch enable pin
as negative level or vice versa . Connect clock to enable pin it will work as FF .

Serial Peripheral Interface

SPI has 4 Port signals

• SCLK - Serial Clock (master O/p)
• MISO - Master Input Slave Output (master I/p)
• MOSI - Master Output Slave Input (Master O/p)
• CS - Chip Select -active low(Master O/p)

During Each SPI clock cycle a full duplex data transmission occur

• Master send a bit on the MOSI line ; slave read it from the same line.
• Slave send a bit on MISO line ;master read it from same line

not all four transmission require all times.

Clock Polarity & Phase (CPOL & CPHA)
CPOL = 0 the base value of clock is 0

• CPHA = 0 data read on clock rising edge & data change on clock falling edge
• CPHA = 1 data read on clock falling edge & data change on clock rising edge

CPOL = 1 the base value of clock is 1

• CPHA = 0 data read on clock falling edge & data change on clock rising edge
• CPHA = 1 data read on clock rising edge & data change on clock falling edge

Clk Divider Circuits

There is a lots of way to design a frequency divider circuit .

Common Methord

Counters & FSM

Design a counter for that even no . The MSB will give expected output .This is the easy way to design a clock divide by any even no circuit .
Main Advantage of this less no of Flops & 50% Duty cycle

Design a FSM for expected output .

Note : You can use counters & FSM for divide by any no circuits,but odd no circuits will not give 50% duty cycle


Condition : input clock must have 50% duty cycle .

Divide By N(Odd No) with 50% Duty Cycle

One clock counter will be Neg edge & other will be pos edge trigger
Condition : N!= 1
Disadvantage : Large No of gate count
Advanatage : 50% Duty cyle ,Simple to design.

FIFO Depth Calculation

We know that FIFO is useful for synchronizing between two clock domains .

In the case of FIFO ,FIFO depth calculation is very important. If we didn't calculate the depth of FIFO properly there is a chance to miss the data( information ).

You can find different equations & solutions from various website .

Here i am trying to explain FIFO depth calculation by a simple example.


Assume that Write Clock Frequency WClk = 200 Mhz
Read Clock Frequency RClk = 100 Mhz
No of data per 100 WClk cycle (Data rate including delay) = 80 data

So

WClk = 200Mhz = 5ns
RClk = 100 Mhz = 10ns

Total Time Taken for writing 80 data = 80*5 = 400ns .
But Time Taken for reading 80 data = 80*10 = 800 ns .

So difference b/w data write & read
same no of data = 800 -400 = 400 ns .

ie we want to store 400ns data to some position.Otherwise data will over write .

How many data will write this 400ns time = 400ns/5ns = 80 .

We have to store this 80 Data .

So we can say that FIFO should have minimum depth of 80 .