Q111. What is the Difference between Magma and Caliber for solving the Problem of DRC LVS?
Magma is an implementation tool, this does only metal level DRC, But Caliber is a sign off tool, it does DRC in POLY and Diffusion level.
Q112. 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.
Q113. What are the violations solved in LVS?
- Open Error
- Short Error
- Device Mismatch
- Port Mismatch
- Instance Mismatch
- Net Mismatch
- Floating Nets
Q114. During power analysis, if you are facing IR drop problem, then how did you avoid?
1. Increase power metal layer width.
2. Go for higher metal layer.
3. Spread macros or standard cells.
4. Provide more straps.
Q115. 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 crosstalk. Cross talk may lead setup or hold violation.
Q116. 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
Q117. 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.
Q118. How spacing helps in reducing crosstalk noise?
width is more ➜ more spacing between two conductors➜ cross coupling capacitance is less ➜ less cross talk
Q119. Why double spacing and multiple vias are used related to clock?
- Why clock? -- because it is the one signal which changes its state regularly and more compared to any other signal. If any other signal switches fast, then also we can use double space.
- Double spacing ➜ width is more ➜ capacitance is less ➜ less cross talk
- Multiple vias ➜ resistance in parallel ➜ less resistance ➜ less RC delay
Q120. 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.
