Cross-Power/Clock-Domain Design for a 1‑bit Control S1 (A → B → A)

Context

You are designing a SoC with two power domains (voltage islands) A and B. A one‑bit control signal S1 originates in domain A and is consumed in domain B. A and B may have different voltages and may or may not share the same clock. Domain B can be power‑gated (off) while A remains on. Design the path(s) and constraints so the system is robust across normal operation, clock-domain crossings (CDC), and power‑gating.

Tasks

1. Normal operation design (A → B): Describe how you would implement S1 crossing from A to B. Specify:
   • Which cells are required (level shifter, isolation, always‑on buffers, etc.).
   • Reset strategy (assert/deassert style, per domain, synchronizers).
   • Where each cell is placed and what powers it.
2. Different clock domains and S1 is timing‑aware (not purely asynchronous): Choose a CDC scheme (two‑flop synchronizer, request/ack handshake, toggle, or async FIFO) and list the STA/CDC constraints you would add (e.g., multicycle, false‑path, max_delay, min_pulse_width). Call out pitfalls to avoid.
3. Same clock domain: How does the design and constraint set change compared with the different‑clock‑domain case? What stays the same due to power/voltage concerns?
4. Round trip A → B → A (S1 feeds through logic in B): How do you close timing/CDC across both crossings? If B can power off, how do you clamp or isolate S1 (to 0, to 1, last value, or high‑Z) and why? Where do you place isolation, what powers it, and how do you handle retention and wake‑up sequencing?