VLSI frequently asked interview question

1. what is the difference between mealy and moore state-machines

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.

2. How to solve setup & Hold violations in the design

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, this will 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]

3. What is antenna Violation & ways to prevent it

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 at-least 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

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