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pureboot: autobaud variant, two versions for review

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Measure the host's bit timing at runtime from a 0xC0 calibration pulse, so one
clock-agnostic image per chip runs at any F_CPU — the RC-oscillator deployments
no longer need a per-clock build.

Two source files, differing only in the write-guard/purity tradeoff:
pureboot_autobaud_pure.cpp (the measured unit in the GPIOR I/O scratch
registers, running-slot write guard dropped, 508 B on the 1284) stays strictly
pure; pureboot_autobaud_reg.cpp (unit in one global register variable, guard
kept, 512 B) keeps every feature at the cost of that single GRV. Both fit
512/510 on all 37 chips and share two licensed simplifications: a slimmed info
block (version + signature; the host derives geometry from the chip database)
and a single-byte activation knock.

pureboot/autobaud.md records the decision, the hand-assembly floor (506 B) that
set the target, and the compiler-knob path to it. Size-tested on every chip via
pureboot_add_autobaud(); the fixed-baud loader is untouched. Sim validation, the
host calibration handshake, and real-hardware acceptance remain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-07-23 22:15:38 +02:00
parent c78bf2841a
commit eecf9673b9
5 changed files with 952 additions and 0 deletions
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35
pureboot/CMakeLists.txt
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@@ -310,3 +310,38 @@ function(pureboot_add_loader name)
set_target_properties(${name} PROPERTIES PUREBOOT_HZ ${PB_CLOCK} PUREBOOT_BAUD ${PB_BAUD}
PUREBOOT_LINK ${_link})
endfunction()
# pureboot_add_autobaud(<name> <source> [RX <pin>] [TX <pin>])
#
# An autobaud software-serial loader from <source> (pureboot_autobaud_*.cpp).
# Autobaud measures the host's bit timing at runtime, so the image carries no
# clock and no baud — one binary per chip runs at any F_CPU. Same per-chip
# geometry, link and codegen flags as pureboot_add_loader(); only the clock and
# baud axes fall away. Two source files are under review (autobaud.md):
# pureboot_autobaud_pure.cpp and pureboot_autobaud_reg.cpp.
function(pureboot_add_autobaud name source)
cmake_parse_arguments(PB "" "RX;TX" "" ${ARGN})
if(NOT PB_RX)
set(PB_RX pb0)
endif()
if(NOT PB_TX)
set(PB_TX pb1)
endif()
foreach(_pin ${PB_RX} ${PB_TX})
if(NOT _pin MATCHES "^p[a-h][0-7]$")
message(FATAL_ERROR "pureboot_add_autobaud(${name}): pin '${_pin}' is not of the form pb1")
endif()
endforeach()
get_property(_base_hex GLOBAL PROPERTY PUREBOOT_BASE_HEX)
get_property(_app GLOBAL PROPERTY PUREBOOT_APP)
get_property(_wrap GLOBAL PROPERTY PUREBOOT_WRAP)
add_executable(${name} ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/${source})
target_link_libraries(${name} PRIVATE libavr)
target_compile_definitions(${name} PRIVATE PUREBOOT_RX=${PB_RX} PUREBOOT_TX=${PB_TX})
target_compile_options(${name} PRIVATE
-fno-ivopts -fira-algorithm=priority -fno-move-loop-invariants -fno-tree-ter -fno-split-wide-types)
target_link_options(${name} PRIVATE -nostartfiles -Wl,--section-start=.text=${_base_hex}
-Wl,--defsym=pureboot_app=${_app} ${_wrap})
add_custom_command(TARGET ${name} POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:${name}>)
endfunction()