Interview - VLSI Design Part 1

Question: what is the difference between Mealy and Moore state machines?

The Moore state machine uses only only entry actions, i.e., output depends only on the previous state. It may has more states and synchronous outputs. The Mealy state machine uses only input actions, i.e., output depends on input and previous state. It may has fewer states and asynchronous outputs. The ouput timing behavior is different. The output of Moore machine has one cycle "delay", whereas the output of Mealy machine is immediately available.

Question: How to solve setup and hold violations in the design?

Setup Time is the amount of time the synchronous input must be stable before the active edge of the clock. Hold Time is the amount of time the synchronous input must be stable after the active edge of the clock.

During the initial iterations only setup violations are fixed whereas hold violations are fixed only after the actual physical place and route information is available. Setup and hold violations are mutually exclusive. Setup violations can be fixed by reducing the combo delay (minimizing the logic level). Hold violations are fixed by increasing the combo delay or by inserting buffers such that it does not cause the setup violations. Increasing or decreasing delays by upsizing or downsizing the cells ripples back into the design and the whole design is to be taken into consideration for carrying out the STA again.

Question: how can you reduce dynamic power?

• Reduce switching activity by designing good RTL.
• Clock gating [Reference] (De-activate the clocks for functions that are not required)
  
• Architectural improvements
• Reduce supply voltage
• Use multi Vdd

Question: What is cross talk?

Switching of the signal in one net can interfere neighboring net due to cross coupling capacitance. Cross talk may lead setup or hold violation.

Question: what are High-Vt and Low-Vt cells?