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vlsi interview 4


VLSI Interview question
what is the difference between mealy and moore state-machines
Ans:

In the mealy state machine we can calculate the next state and output both from the input and state. But in the moore state machine we can calculate only next state but not output from the input and state and the output is issued according to next state.

How to solve setup & Hold violations in the design
Ans:
To solve setup violation
1. optimizing/restructuring combination logic between the flops.
2. Tweak flops to offer lesser setup delay [DFFX1 -> DFFXx]
3. Tweak launch-flop to have better slew at the clock pin, thiswill make CK->Q of launch flop to be fast there by helping fixing setup violations
4. Play with skew [ tweak clock network delay, slow-down clock to capturing flop and fasten the clock to launch-flop](otherwise called as Useful-skews)

To solve Hold Violations

1. Adding delay/buffer[as buffer offers lesser delay, we go for spl Delay cells whose functionality Y=A, but with more delay]
2. Making the launch flop clock reaching delayed
3. Also, one can add lockup-latches [in cases where the hold time requirement is very huge, basically to avoid data slip]

What is antenna Violation & ways to prevent it
Ans:

During the process of plasma etching, charges accumulate along the metal strips. The longer the strips are, the more charges are accumulated. IF a small transistor gate connected to these long metal strips, the gate oxide can be destroyed (large electric field over a very thin electric) , This is called as Antenna violation.

The ways to prevent is , by making jogging the metal line, which is atleast one metal above the layer to be protected. If we want to remove antenna violation in metal2 then need to jog it in metal3 not in metal1. The reason being while we are etching metal2, metal3 layer is not laid out. So the two pieces of metal2 got disconnected. Only the piece of metal connected to gate have charge to gate. When we laydown metal3, the remaining portion of metal got charge added to metal3. This is called accumulative antenna effect.
Another way of preventing is adding reverse Diodes at the gates

what is tie-high and tie-low cells and where it is used
Ans:

Tie-high and Tie-Low cells are used to connect the gate of the transistor to either power or ground. In deep sub micron processes, if the gate is connected to power/ground the transistor might be turned on/off due to power or ground bounce. The suggestion from foundry is to use tie cells for this purpose. These cells are part of standard-cell library. The cells which require Vdd, comes and connect to Tie high…(so tie high is a power supply cell)…while the cells which wants Vss connects itself to Tie-low.

what is the difference between latches and flip-flops based designs
Ans:

Latches are level-sensitive and flip-flops are edge sensitive. latch based design and flop based design is that latch allowes time borrowing which a tradition flop does not. That makes latch based design more efficient. But at the same time, latch based design is more complicated and has more
issues in min timing (races). Its STA with time borrowing in deep pipelining can be quite complex.

What is High-Vt and Low-Vt cells.
Ans:

Hvt cells are MOS devices with less leakage due to high Vt but they have higher delay than low VT, where as the low Vt cells are devices which have less delay but leakage is high. The thereshold(t) vloltage dictates the transistor switching speed , it matters how much minimum threshold voltage applied can make the transistor switching to active state which results to how fast we can switch the trasistor. disadvantage is it needs to maintain the transistor in a minimum subthreshold voltage level to make ir switch fast so it leads to leakage of current inturn loss of power.

What is LEF mean?
Ans:

LEF is an ASCII data format from Cadence Design inc, to describe a standard cell library. It includes the design rules for routing and the Abstract layout of the cells. LEF file contains the following,

Technology: layer, design rules, via-definitions, metal-capacitance

Site : Site extension

Macros : cell descriptions, cell dimensions, layout of pins and blockages, capacitances

what is DEF mean?
Ans:

DEF is an ASCII data format from Cadence Design inc., to describe Design related information.

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