Q21. What Is the Importance of IR Drop Analysis?
IR drop determines the level of voltage at the pins of standard cells. Value of acceptable IR drop will be decided at the start of the project and it is one of the factors used to determine the derate value. If the value of IR drop is more than the acceptable value, it calls to change the derate value. Without this change, timing calculation becomes optimistic.
For example setup slack calculated by the tool is less than the reality.
Q22. If You Have Both IR Drop and Congestion How Will You Fix It?
- Spread macros
- Spread standard cells
- Increase strap width
- Increase number of straps
- Use proper blockage
Q23. In A Reg to Reg Path If You Have Setup Problem Where Will You Insert Buffer-near to Launching Flop or Capture Flop? Why?
- (buffers are inserted for fixing fanout violations and hence they reduce setup violation; otherwise we try to fix setup violation with the sizing of cells; now just assume that you must insert buffer!)
- Near to capture path.
- Because there may be other paths passing through or originating from the flop nearer to launch flop. Hence buffer insertion may affect other paths also. It may improve all those paths or degrade. If all those paths have violation, then you may insert buffer nearer to launch flop provided it improves slack.
Q24. Why Buffers Are Used in Clock Tree?
To balance skew (i.e. flop to flop delay)
Q25. What Is Cross Talk?
Switching of the signal in one net can interfere neighboring net due to cross coupling capacitance. This affect is known as cross talk. Cross talk may lead setup or hold violation.
Q26. How Can You Avoid Cross Talk?
- Double spacing → more spacing → less capacitance → less cross talk
- Multiple vias → less resistance → less RC delay
- Shielding → constant cross coupling capacitance → known value of crosstalk
- Buffer insertion → boost the victim strength.
Q27. How Shielding Avoids Crosstalk Problem? What Exactly Happens There?
- High frequency noise (or glitch) is coupled to VSS (or VDD) since shielded layers are connected to either VDD or VSS.
- Coupling capacitance remains constant with VDD or VSS.
Q28. How Spacing Helps in Reducing Crosstalk Noise?
width is more → more spacing between two conductors → cross coupling capacitance is less → less cross talk.
Q29. How Buffer Can Be Used in Victim to Avoid Crosstalk?
Buffer increase victims signal strength; buffers break the net length → victims are more tolerant to coupled signal from aggressor.
Q30. Why Setup is checked at max corner and Hold at min corner?
For setup, the required time should be more than the arrival time. And setup violates when arrival time is more. So, setup check is more pessimistic when arrival time is more or when the launch clock reaches late than the capture clock. That means delay is more. So, setup will be checked at max delays.
For hold, the arrival time should be more than the required time. And hold violates when required time is more. So, hold check is more pessimistic when required time is more or when the launch clock reaches early than the capture clock. That means data arrival time is less. So, hold will be checked at min delays.
