Power Management

Which correctly pairs a caveat of power-gated blocks with its countermeasure?

  1. AContents always survive power-off, so nothing is needed
  2. BFlip-flop contents are lost, so needed state is saved in retention registers or external memory before power-off
  3. CPower-off doubles the voltage of other blocks
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Answer: B. Flip-flop contents are lost, so needed state is saved in retention registers or external memory before power-off

Flip-flops and SRAM in a powered-off block lose their contents, so to resume where it left off, state must be saved before cutting power. Options include retention registers, which pair each flip-flop with a small always-powered storage element, or software writing state out to memory. Saving, restoring and powering up cost time and energy, so for short idle periods it may be better not to cut power.

  • A. Ordinary flip-flops lose their contents when unpowered.
  • B. Correct. Wake-up time and energy must also be considered.
  • C. Other blocks' voltages do not change.
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More Power Management questions

  1. Which correctly compares clock gating and power gating?
  2. In power gating, why are a block's power switches often turned on gradually in sequence rather than all at once?
  3. What is the role of the isolation cells placed between a powered-off block and running blocks?
  4. What is the role of a level shifter inserted when passing signals between two blocks running at different supply voltages?
  5. What is the main purpose of dividing an SoC into multiple power domains?