docs: add e-ink refresh-latency curve and boot-time budget notes
This commit is contained in:
Julien Calixte
@@ -10,3 +10,4 @@
|
||||
| Curve | What it decides |
|
||||
| --- | --- |
|
||||
| [`wifi-auto-sync.md`](wifi-auto-sync.md) | `auto_sync` interval vs Wi-Fi energy (a `1/T` hyperbola) — why the default is 10 min and opportunistic, not a wall-clock timer. |
|
||||
| [`epd-refresh-latency.md`](epd-refresh-latency.md) | E-ink refresh latency vs rows driven — the full / full-area-partial / windowed-Y cost model behind typing responsiveness and the boot splash→editor swap. |
|
||||
|
||||
117
docs/tradeoff-curves/epd-refresh-latency.md
Normal file
117
docs/tradeoff-curves/epd-refresh-latency.md
Normal file
@@ -0,0 +1,117 @@
|
||||
# E-ink refresh latency vs rows driven
|
||||
|
||||
> **Model:** on this GDEY0579T93 (SSD1683 dual-controller) panel, refresh time is
|
||||
> set by **how many gate lines (rows, the Y axis) are driven** and **which
|
||||
> waveform LUT runs** — *not* by the pixel column width. Three refresh modes fall
|
||||
> out of that: full refresh (~1870 ms), full-area partial (~630 ms), and
|
||||
> windowed-Y partial (~100–130 ms for one text line). This is the cost model
|
||||
> behind per-keystroke typing, the boot splash→editor swap
|
||||
> ([`../notes/boot-time-budget.md`](../notes/boot-time-budget.md)), and the
|
||||
> scroll/gutter spikes.
|
||||
>
|
||||
> Tradeoff-curves index: [`README.md`](README.md). Docs index:
|
||||
> [`../README.md`](../README.md). Driver:
|
||||
> [`../../firmware/src/epd.rs`](../../firmware/src/epd.rs). Bench origin:
|
||||
> [`../spikes.md`](../spikes.md) (Spikes 5 + 8).
|
||||
|
||||
## The model
|
||||
|
||||
A refresh is three serial costs:
|
||||
|
||||
```
|
||||
set RAM window → clock the pixels out over SPI → run the update waveform
|
||||
(fixed) (scales with bytes = rows) (scales with gate lines
|
||||
AND with which LUT)
|
||||
```
|
||||
|
||||
Two things do **not** help, and one thing does:
|
||||
|
||||
- **Column width (X) is free to keep full.** The panel is a master (`0x00`) +
|
||||
slave (`0x80`) pair with the framebuffer split at the seam; every refresh
|
||||
drives *both* controllers full width so the seam/mirror math stays intact
|
||||
(`update_part` / `write_frame_bank` in
|
||||
[`epd.rs`](../../firmware/src/epd.rs)). Narrowing the *column* saves nothing —
|
||||
"the waveform time dominates, not the data clock-out."
|
||||
- **Row count (Y) is the knob.** E-paper drive time scales with the number of
|
||||
gate lines transitioned, so restricting the refresh to a horizontal band of
|
||||
rows is the real win — a one-line band is far cheaper than the whole panel.
|
||||
- **The LUT sets the tier.** A *partial* update (`0x22`←`0xFF`) runs a short
|
||||
waveform that only nudges pixels that changed. A *full* update (`0x22`←`0xD7`,
|
||||
the fast-full LUT) runs ~3× as many frames per pixel to fully clear-and-set —
|
||||
slower, but it erases ghosting and re-establishes a known image.
|
||||
|
||||
Within partial mode the latency is roughly linear in rows:
|
||||
|
||||
```
|
||||
t_partial(rows) ≈ 90 ms + 2 ms · rows
|
||||
└ floor ┘ └ per-gate-line slope ┘
|
||||
```
|
||||
|
||||
Floor ≈ fixed SPI setup + border/VCOM commands + waveform ramp; slope ≈ the
|
||||
per-row waveform drive. Full refresh sits in its own flat tier (~1870 ms, all 272
|
||||
rows, full-clear LUT) and **cannot be windowed** — the clear waveform needs the
|
||||
whole panel.
|
||||
|
||||
## The curve
|
||||
|
||||
```
|
||||
Partial-refresh latency vs rows driven (Y-band) t ≈ 90 ms + 2 ms · rows
|
||||
|
||||
ms
|
||||
1870 |========================================= FULL refresh — separate tier:
|
||||
| all 272 rows, full-clear LUT,
|
||||
| un-windowable. ~3× the partial
|
||||
| waveform. De-ghosts + seeds a
|
||||
| known image.
|
||||
|
|
||||
630 | * ← full-area partial (272 rows)
|
||||
| . ·
|
||||
| . ·
|
||||
| . · slope ≈ 2 ms / gate line
|
||||
300 | . ·
|
||||
| . ·
|
||||
110 | * ← one text line (~10 rows): the per-keystroke path
|
||||
90 | * ← floor (SPI setup + border/VCOM + waveform ramp)
|
||||
+----+----+----+----+----+----+----+----+----+----+----+---
|
||||
0 25 50 75 100 125 150 175 200 225 250 272 rows
|
||||
```
|
||||
|
||||
| Mode | Rows driven | Latency | LUT | Used for |
|
||||
| --- | ---: | ---: | --- | --- |
|
||||
| Full refresh | 272 (all) | **~1870 ms** (measured) | full-clear | first cold-boot image; periodic de-ghost (every `FULL_REFRESH_EVERY` = 64 updates) |
|
||||
| Full-area partial | 272 | **~630 ms** (measured; 680 ms at boot) | partial | deletes, caret moves, mode switches, the snackbar, and the boot splash→editor swap |
|
||||
| Windowed-Y partial | ~10 (1 line) | **~100–130 ms** (estimated¹) | partial | additive per-keystroke typing |
|
||||
|
||||
¹ The single-line windowed figure is projected from the floor+slope model and the
|
||||
Spike 5 full-area measurement; the exact bench number is still to be confirmed
|
||||
from the on-device refresh log (`{mode} refresh #N … {ms} ms` in
|
||||
[`main.rs`](../../firmware/src/main.rs)). The 1870 ms full and 630 ms full-area
|
||||
figures are measured.
|
||||
|
||||
## Two things that bound it
|
||||
|
||||
**Ghosting caps the partial streak.** Partial updates leave faint residue, so a
|
||||
full refresh every 64 updates resets clarity and panel state. You can't "always
|
||||
partial" — the ~1870 ms tier is a periodic tax paid for longevity, not a mode you
|
||||
can retire.
|
||||
|
||||
**The first cold-boot image must be a full refresh.** After power-on the `0x26`
|
||||
"previous" bank holds garbage, and a partial refresh *diffs against it* — so the
|
||||
very first clean paint has to be the full tier. This is why boot pays exactly one
|
||||
unavoidable ~1.9 s full refresh, and why the splash (which rides it) is nearly
|
||||
free while the *editor's* first frame can be a cheap partial on top. Full
|
||||
derivation: [`../notes/boot-time-budget.md`](../notes/boot-time-budget.md).
|
||||
|
||||
## What it decides
|
||||
|
||||
- **Per-keystroke typing → windowed-Y partial.** Only the touched line's band is
|
||||
driven (~100–130 ms), so typing keeps up with the keyboard; the panel never
|
||||
repaints per keystroke off that line.
|
||||
- **Boot splash→editor → full-area partial (~630 ms), not a second full refresh
|
||||
(~1870 ms).** The splash already seeded the baseline, so the editor rides in on
|
||||
a partial — the ~1.25 s cold-boot win recorded in the boot-time budget.
|
||||
- **Deletes, caret moves, mode flips, snackbar → full-area partial.** These erase
|
||||
ink or change the panel off the cursor line, which a windowed band would ghost,
|
||||
so they take the whole-panel partial rather than a windowed one.
|
||||
- **Splash + periodic → full refresh.** The unavoidable first image and the
|
||||
every-64 de-ghost.
|
||||
Reference in New Issue
Block a user