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pureboot: correct the 1284P deployment profiles in the README

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The deployment section claimed the 1284P has no standalone profile, runs
BOOTSZ = 512 words always, and self-updates with no fuse change, reset
landing at 0x1f800 — internally contradictory (a 512-word section starts
at 0x1fc00, and the section holding both 1 KiB slots is 1024 words) and
contradicted by update_preflight, which refuses a self-update unless the
boot section covers two slots. The text described a 512-byte-slot
geometry this chip's loader cannot have. In truth the 328P profile table
maps onto the 1284P doubled: standalone = 512 words (the smallest
section is exactly the 1 KiB slot, reset at the loader base), self-update
= 1024 words with the loader-first reset walking the staging slot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-07-21 16:48:46 +02:00
parent 65f8fdd537
commit b7d082ff11
1 changed files with 10 additions and 11 deletions
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21
pureboot/README.md
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@@ -108,17 +108,16 @@ write `0xff` (per page for flash, per byte for EEPROM).
## Deployment
**Megas**: program the loader at `flash − 512` with an external programmer.
Every mega's smallest-but-one BOOTSZ puts the boot-section start exactly at
the loader base (512 B — the m8/16/168A reach it at their second-smallest
step, the m32/328P at their smallest), so the ATmega328P profiles below
apply to all of them with their own addresses; the per-chip BOOTSZ ladders
live in the host tool (`BOOT_FUSE`). The **ATmega1284P** is the exception:
its smallest boot section is 1 KB, so the standalone profile does not exist
— BOOTSZ = 512 words always, and with BOOTRST programmed reset lands at
0x1f800, one erased slot below the loader (the loader-first walk behavior
below, built in). Its staging slot sits inside that same 1 KB section, so
self-update needs no fuse change.
**Megas**: program the loader at `flash − slot` with an external programmer.
Every mega has a BOOTSZ step whose boot section is exactly the loader slot —
512 B, the m8/16/168A's second-smallest step, the m32/328P's smallest; on
the m1284P that step is its smallest, 512 words, which is why its slot is
1 KiB — so the ATmega328P profiles below apply to every mega with its own
addresses and slot size; the per-chip BOOTSZ ladders live in the host tool
(`BOOT_FUSE`). The 1284P's numbers: standalone = BOOTSZ 512 words (reset at
the loader base 0x1fc00); self-update = 1024 words, covering both 1 KiB
slots, the loader-first reset landing at 0x1f800 — the staging slot, walked
across when erased.
ATmega328P profiles (addresses for its 32 KiB):