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ADR-0009 — E-paper refresh-mode discipline

Status: accepted Source: spec 014 (display); NFR-003; FR-094; components/cdc_hal/include/cdc_hal/IDisplay.h (RefreshMode)

Context

Section titled “Context”

The 2.9” monochrome e-paper updates only on content change. RefreshMode has three values: FULL (slow, clears ghosting), PARTIAL (fast, may ghost; periodically promoted to FULL to clear ghosting), and PARTIAL_LIGHT (partial refresh that is never promoted to FULL, for tiny low-churn updates such as the lock-screen clock). An async flush must carry the full RefreshMode; collapsing PARTIAL_LIGHT into PARTIAL would reintroduce periodic full refreshes on the lock-screen clock.

Decision

Section titled “Decision”

The display pipeline preserves refresh-mode discipline. The lock-screen clock uses PARTIAL_LIGHT, and PARTIAL_LIGHT is never promoted to a full refresh.

  • An async flush carries the full RefreshMode end-to-end; it is not downgraded or collapsed.
  • PARTIAL may be periodically promoted to FULL to clear ghosting; PARTIAL_LIGHT may not.
  • The busy-wait must outlast the full-refresh waveform so back-to-back full refreshes do not abort each other.

Consequences

Section titled “Consequences”
  • Enables: a low-churn lock-screen clock that updates without the visual disruption and wear of repeated full refreshes.
  • Must hold: any flush path that forwards a refresh mode must forward PARTIAL_LIGHT unchanged; the per-second clock tick stays PARTIAL_LIGHT.
  • Cost: PARTIAL_LIGHT regions accumulate ghosting over time because they are never cleared by a full refresh; only content that tolerates this should use it.