Retention mechanisms in power gated designs

How can we retain state of some of the registers in the design? How to deal with memory state?

Let me try to explain each one of them based on my recent design experience.
For regular logic blocks, there are multiple ways to wake-up faster without losing much of information.

  • Use retention flops to save state of some important registers. For example state of control block, which forms the heart of the whole system.
  • If the chip is aimed for At-Speed testing, scan chains of the design can be used to scan out the data to an external memory and scan in after wake-up. This may not be as fast as using retention flops.
  • ….. there are many more possible methods.

Again w.r.t to retention flops there were questions about, How many type of retention flops are available.
I have seen 3 types.

  1. Single save/restore pin retention latch (Slave latch being always on)
  2. Single pin balloon Latch
  3. Dual Pin balloon Latch
Pro's and Con's of Single Pin Vs Dual Pin retention flops:

Advantages of Single Pin:

  • Minimal area impact
  • Single signal controls retention

Disadvantages of Single Pin:

  • Performance Impact on the register
  • Hold Time requirements for the input data

Advantage of Dual Pin:

  • Minimal leakage power
  • Minimal performance impact compared to the Single Pin design
  • Minimal dependency on the clock for the control signals.

Disadvantages of Dual Pin:

  • Area Impact
  • More Complex System Design
  • More Buffer Network and AON network required.

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