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# =Electronics Hardware Questions=

• Two capacitors are connected in parallel through a switch. C1= 1uF, C2= 0.25uF.
Initially the switch is open, C1 is charged to 10V. What happens if we close the switch? No losses in wires and capacitors.

• You have 2 switches to control the light in the long corridor. You want to be able to turn the light on entering the corridor and turn it off at the other end. Do the wiring circuit.

• There are 3 switches that can turn on and off a light in the room. How to connect them?

• What will be the voltage level between the two capacitors? The Vcc = 10v DC.
Sent by Tanh, VLSI engineer

• Suppose, you work on a specification for a system with some digital parameters. The spec table has has Min,Typ and Max colomns for each parameter. In what column would you put a Setup time and a Hold time?

•   Design a simple circuit based on combinational logic to double the output frequency.

• 8bit ADC with parallel output converts an input signal into digital numbers. You have to come up with an idea of a circuit , that finds MAX  of every 10 numbers at the output of the ADC.

• Implement a comparator that compares two 2-bit numbers A and B. The comparator should have 3 outputs:  A > B, A < B, A = B. Do it two ways:
- using combinational logic;
- using multiplexers. Write HDL code for your schematic at RTL and gate level.

• You have a 8 bit ADC clocking data out every  1mS.  Design a system that  will sort the output data and keep the statistics of how often each binary number appears at the output of ADC.

• What types of flip-flops do you know?

• Implement D- latch from
- RS flip flop;
- multiplexer.

• How to convert D-latch into JK-latch and JK-latch into D-latch?

• There are two counters to 16, built from negedge D- FF . The first circuit is synchronous and the second one is "ripple" (cascading). Which circuit has less propagation delay?

• What is the difference between a flip-flop and a latch?
Write an HDL code for their behavioral models.

• Describe the operation of a DAC. What are the most important parameters of a DAC? Do we really need both INL and DNL to estimate linearity?

• Compare  briefly all types of ADC that you know .

• How will the output signal of an ideal integrator look like after
- a positive pulse is applied to the input;
- a series of 10 positive pulses ?

• How to design a divide-by-3 counter with equal duty cycle ?
question from Anonymous

• For an 8-bit flash A/D converter with an input range from 0V to 2.55V, describe what happens when the input voltage changes from 1.27V to 1.28V

• Your system has CPU, ALU and two 8bit registers. There is no external memory. Can you swap the contence of the registers ?

• We swapped 2 transistors in CMOS inverter (put n-transistor at the top and p-transistor at the bottom). Can this circuit work as a non-inverting buffer?
(By E.Martovetsky,design eng)

• Convert D-latch into divider by 2.
What is the max clock frequency  the circuit can handle ?
T_setup= 6nS
T_hold = 2nS
T_propagation = 10nS
Hint from Hitequest

• The circle can rotate clockwise and back. Use minimum hardware to build a circuit to indicate the direction of rotating.

Hint from Hitequest

• Provide 2-dimentional plot of how the output of digital circuit will look like, if on axis X we sweep the clock period, while on axis Y we sweep the data setup time (Tclk vs Tsetup).

• For chip design/test/product engineers :
An IC device draws higher current when temperature gets:
- higher?
- lower?
Hint from Hitequest

• To enter the office people have to pass through the corridor. Once someone gets into the office the light turns on. It goes off when noone is present in the room. There are two registration sensors in the corridor.  Build a state machine diagram and design a circuit to control the light.

• A voltage source with internal impedance Z_source = 50 OHm is connected to a transmission line with Z = 50 OHm. Z_load is also 50 OHm.
The voltage source generates a single voltage step 1V.
What will be the voltage level on the load:

a)  2V , because the reflected signal will be in-phase with the incident signal;
b)  0.33V , because the voltage is devided between Z_source , Z_load and Z_transm.line;
c)  0.5V , because the voltage is devided between Z_source and Z_load.

Hint from Hitequest

• Draw a transistor schematic of NOR gate,it's layout and a cross section of the layout.
This question is quite popular on interviews.

• The silicon of a new device has memory leak. When all "0" are written into RAM, it reads back all "0" whithout any problem. When all "1" are written, only 80% of memory cells are read back correctly. What can be possibly the problem with the RAM?
M.Altshuler, product engineer.

• Draw a CMOS inverter. Why does CMOS technology dominate in VLSI manufacturing?
L.Backer, DFT engineer

• Design a FIFO 1 byte wide and 13 words deep. The FIFO is interfacing 2 blocks with different clocks. On the rising edge of clk the FIFO stores data and increments wptr. On the rising edge of clkb the data is put on the b-output,the rptr points to the next data to be read.
If the FIFO is empty, the b-output data is not valid. When the FIFO is full the existing data should not be overriden.
When rst_N is asserted, the FIFO pointers are asynchronously reset.

module fifo1 (full,empty,clk,clkb,ain,bout,rst_N)
output [7:0] bout;
input [7:0] ain;
input clk,clkb,rst_N;
output empty, full;
reg [3:0] wptr, rptr;
...

endmodule

Hint from Hitequest

• What does CMOS stand for? VLSI? ASIC?
This was in the series of quick questions in the interview at Analog Devices. We use these abbreviations daily, but not everyone remembers what they stand for.
Hint from Hitequest

• Design a COMBINATIONAL circuit that can divide the clock frequency by 2.

• Design a 2bit up/down counter with clear using gates. (No verilog or vhdl)

• We have a circular wheel with half painted black and the other half painted white. There are 2 censors mounted 45 degree apart at the surface of this wheel( not touching the wheel) which give a "1" for black and "0" for white passing under them. Design a circuit to detect which way the wheel is moving. Can not assume any fixed position for start.

• We have a FIFO which clocks data in at 100mhz and clocks data out at 80mhz. On the input there are only 80 data bits in any order during each 100 clocks. In other words, a 100 input clock will carry only 80 data bits, and the other twenty clocks carry no data (data is scattered in any order). How big the FIFO needs to be to avoid data over/under-run.

• Instead of specifying SETUP and HOLD time, can we just specify a SETUP time for '1' and a SETUP time for '0'?

• Here some hardware digital design specific questions, offered by Suhas:
(1) When will you use a latch and a flipflop in a sequential design?
(2) Design a 1-bit fulladder using a decoder and 2 "or" gates?
(3) You have a circuit operating at 20 MHz and 5 volt supply. What would you do to reduce the power consumption in the circuit- reduce the operating frequency of 20Mhz or reduce the power supply of 5Volts and why?
(4) In a nmos transistor, how does the current flows from drain to source in saturation region when the channel is pinched off?
(5) In a SRAM circuit, how do you design the precharge and how do you size it?
(6) In a PLL, what elements(like XOR gates or Flipflops) can be used to design the phase detector?
(7) While synthesis of a design using synopsys design compiler, why do you specify input and output delays?
(8) What difference do you see in the timing reports for a propogated clock and an ideal clock?
(9) What is timeborrowing related to Static timing anaylsis in Primetime?

• What is the purpose of a diode next to relay on schematics?

• Design a combinational circuit, that multiplies an input decimal digit represented in BCD by 5. The output is to be represented in BCD. Show that the outputs can be obtained from the input lines without using any logic gates.

• I got this question on the interview to the company that makes flash memory controllers. They let me take it home and think for a few days. A.M.
There is a system with 4 flash memory banks. When data comes to the system, it will be randomly sent to one of the 4 banks. The system has a feature called Native Command Queuing (NCQ) which allows the host to send multiple commands at a time.
The number of commands sent at a time is called the queue depth. With a queue depth of 1, only 1 of 4 memory banks will be active.
With a queue depth of 2, we expect 2 of 4 flash memory banks to be active except in the case when both commands are to the same bank.
The question is, what is the expected number of flash memory banks that would be active, given queue depths of:
2, 4, 8, and 16 ?

• R.K
How do you synchronize different CLK Speeds in a system?

• R.K
Two N-Channel MOSFETs are connected in series and are connected between Vcc and GND. Will turning-on the top MOSFET turn on the BOTTOM one? If yes, Why?
What is a solution to this problem?

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