Friday, September 5, 2008

VLSI Interview Questions

what are the differences between SIMULATION and SYNTHESIS

Simulation <= verify your design.
synthesis <= Check for your timing
Simulation is used to verify the functionality of the circuit.. a)Functional Simulation:study of ckt's operation independent of timing parameters and gate delays. b) Timing Simulation :study including estimated delays, verify setup,hold and other timing requirements of devices like flip flops are met.
Synthesis:One of the foremost in back end steps where by synthesizing is nothing but converting VHDL or VERILOG description to a set of primitives(equations as in CPLD) or components(as in FPGA'S)to fit into the target technology.Basically the synthesis tools convert the design description into equations or components

Can u tell me the differences between latches & flipflops?

There are 2 types of circuits:
1. Combinational
2. Sequential

Latches and flipflops both come under the category of "sequential circuits", whose output depends not only on the current inputs, but also on previous inputs and outputs.

Difference: Latches are level-sensitive, whereas, FF are edge sensitive. By edge sensitive, I mean O/p changes only when there is a clock transition.( from 1 to 0, or from 0 to 1)

Example: In a flipflop, inputs have arrived on the input lines at time= 2 seconds. But, output won't change immediately. At time = 3 seconds, clock transition takes place. After that, O/P will change.
Flip-flops are of 2 types:
1.Positive edge triggered
2. negative edge triggered

1)fllipflops take twice the nymber of gates as latches
2) so automatically delay is more for flipflops
3)power consumption is also more

latch does not have a clock signal, whereas a flip-flop always does.

What is slack?

The slack is the time delay difference from the expected delay(1/clock) to the actual delay in a particular path.
Slack may be +ve or -ve.

Equivalence between VHDL and C?

There is concept of understanding in C there is structure.Based upon requirement structure provide facility to store collection of different data types.

In VHDL we have direct access to memory so instead of using pointer in C (and member of structure) we can write interface store data in memory and access it.

RTL and Behavioral

Register transfer language means there should be data flow between two registers and logic is in between them for end registers data should flow.

Behavioral means how hardware behave determine the exact way it works we write using HDL syntax.For complex projects it is better mixed approach or more behavioral is used.

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