Interview question - Fabrication, Hypothesis, Power, Process

While you are eating lunch at your regular table in the company cafeteria, a vice president sits down and starts to talk about the difficulties with a new chip. The chip is a slight modification of existing design that has been ported to a new fabrication process. Earlier that day, the first sample chips came back from fabrication. The good news is that the chips appear to function correctly. The bad news is that they consume about 10% more power than had been predicted. The vice president explains that the extra power consumption is a very serious problem, because power is the most important design metric for this chip. The vice president asks you if you have any idea of what might cause the chips to consume more power than predicted
  1. Hypothesize a likely cause for the surprisingly large power consumption, and justify why your hypothesis is likely to be correct.
  2. Briefly describe how to determine if your hypothesized cause is the real cause of the surprisingly large power consumption.
  3. The vice president wants to get the chips out to market quickly and asks you if you have any ideas for reducing their power without changing the design or fabrication process. Describe your ideas, or explain why her suggestion is infeasible.

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  1. I would ask the vice president how chip power has been estimated. The only accurate way to estimate power is to get the chip nodes activity from a set of functional pattern and then use a power calculator. If real power is higher than expected, so power estimation has probably not been done correctly.

    To reduce power without any redesign ... hum maybe reducing power supply or clock frequency if it make sens ...

  2. Leakage current — because same design, but different fabrication process resulted in power change.

    Measure power consumption of circuit with clock turned off, if higher than
    expected, then leakage current is the problem.

    And, Reduce the clock frequency to save power if possible.


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