build: the libavr pin advances over the audit rounds

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -66,16 +66,18 @@ function(add_image_outputs name)
 	                           $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.bin)
 endfunction()
 
-# The TinySafeBoot protocol reimplemented on libavr in three variants that trade
+# The TinySafeBoot protocol reimplemented on libavr in variants that trade
 # clarity for size. Each links into the ATmega328P boot section (BOOTSZ selects
 # its size; BOOTRST vectors a reset to its base) with -nostartfiles — a polled
-# loader has no use for the crt or the vector table. The naked entry sits in
-# .vectors, laid first, and runs. The boot base is FLASHEND+1 minus the section
-# size; the linker section-start and the source's boot_bytes agree. tsb_app is
+# loader has no use for the crt or the vector table. The entry sits in
+# .vectors, laid first, and runs — avr::startup::entry on the policy tier,
+# the experiment tiers' own naked stubs elsewhere, each documented in its
+# source. The boot base is FLASHEND+1 minus the section size; the linker
+# section-start and the source's boot_bytes agree. tsb_app is
 # the application's reset vector, pinned to 0 here so the loaders jump to a
 # named function; --pmem-wrap-around lets relaxation turn that absolute jump
 # into the wrapped rjmp AVR's modulo-flash PC actually executes.
-# All three implement the full oracle feature set (see oracle/README.md):
+# All four implement the full oracle feature set (see oracle/README.md):
 # watchdog bail, one-wire half-duplex, config-page activation timeout, password
 # gate, emergency erase, config/flash/EEPROM read-write. They differ only in how,
 # and the size gradient is the cost of that "how" — see dev/lessons.md.
@@ -167,6 +169,14 @@ if(PROJECT_IS_TOP_LEVEL)
 		add_test(NAME pureboot.planner
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_planner.py
 		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py)
+		add_test(NAME pureboot.scan
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_scan.py
+		                 ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py)
+		# CMakePresets.json is generated; hand edits drift the moment the
+		# generator reruns, so the gate holds the pair together.
+		add_test(NAME presets.generated
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/tools/make_presets.py
+		                 --check)
 		add_test(NAME pureboot.handshake
 		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/test_handshake.py)
 		add_test(NAME pureboot.updatelink
@@ -270,6 +280,12 @@ if(PROJECT_IS_TOP_LEVEL)
 	add_test(NAME pureboot_autobaud.size
 	         COMMAND ${CMAKE_COMMAND} -DSIZE_TOOL=${CMAKE_SIZE} -DELF=$<TARGET_FILE:pureboot_autobaud>
 	                 -DLIMIT=${PUREBOOT_LIMIT} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_size.cmake)
+	# The measured unit is the loader's only RAM object and sits at the very
+	# start of SRAM — where the host reads the bit period from (--info's
+	# measured clock), so the address is wire contract, not layout accident.
+	add_test(NAME pureboot_autobaud.unit
+	         COMMAND ${CMAKE_COMMAND} -DOBJDUMP=${CMAKE_OBJDUMP} -DELF=$<TARGET_FILE:pureboot_autobaud>
+	                 -DRAM_START=${PUREBOOT_RAM_START} -P ${CMAKE_CURRENT_SOURCE_DIR}/test/check_unit.cmake)
 
 	# One point of the exhaustive matrix, named from its resolved parameters
 	# so the enumeration cannot collide with itself. `pins` is empty for the
@@ -390,6 +406,38 @@ if(PROJECT_IS_TOP_LEVEL)
 		                      RX ${PUREBOOT_USART1_RX} TX ${PUREBOOT_USART1_TX})
 	endif()
 
+	# The OSCCAL axis at its fixed points: the stock shape, and the tightest
+	# image in the space with the trim on top — the axis adds one register
+	# write, and these points hold both of its addressing encodings to every
+	# chip's budget.
+	pureboot_size_variant(pureboot_osccal OSCCAL 0x9c)
+	pureboot_size_variant(pureboot_autobaud_osccal SERIAL autobaud OSCCAL 0x9c)
+	if(PUREBOOT_HAS_USART)
+		pureboot_size_variant(pureboot_autobaud_osccal_on_usart0 SERIAL autobaud OSCCAL 0x9c
+		                      RX ${PUREBOOT_USART0_RX} TX ${PUREBOOT_USART0_TX})
+	endif()
+
+	# The trim byte, observed through the wire from the first prompt — one
+	# chip per OSCCAL addressing class: extended I/O on the 328P (data 0x66,
+	# an sts — DS40002061B §36), plain I/O on the 85 (data 0x51, an out —
+	# Atmel-2586 §21).
+	if(LIBAVR_MCU MATCHES "^(atmega328p|attiny85)$" AND DEFINED PB_DEVICE)
+		if(LIBAVR_MCU STREQUAL "atmega328p")
+			set(_osccal_addr 0x66)
+		else()
+			set(_osccal_addr 0x51)
+		endif()
+		get_target_property(_osccal_hz pureboot_osccal PUREBOOT_HZ)
+		get_target_property(_osccal_baud pureboot_osccal PUREBOOT_BAUD)
+		add_test(NAME pureboot.osccal
+		         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/test/pbosccal.py
+		                 ${PB_DEVICE} $<TARGET_FILE:pureboot_osccal> ${PUREBOOT_SIM_MCU}
+		                 ${_osccal_hz} ${PUREBOOT_BASE_HEX} ${PUREBOOT_PAGE} ${_osccal_baud}
+		                 ${_osccal_addr} 0x9c ${CMAKE_CURRENT_SOURCE_DIR}/pureboot/pureboot.py
+		                 ${CMAKE_BINARY_DIR}/pbosccal-work)
+		set_tests_properties(pureboot.osccal PROPERTIES TIMEOUT 120)
+	endif()
+
 	# One configured deployment end to end — a real board's shape rather
 	# than the stock assumption: the ATmega328P on its shipped 1 MHz fuses,
 	# the software UART on hand-picked pins (TX = PB1, RX = PB5), the ladder
@@ -468,9 +516,9 @@ if(PROJECT_IS_TOP_LEVEL)
 		set_tests_properties(pureboot.usart1 PROPERTIES TIMEOUT 180)
 	endif()
 
-	# The autobaud variants driven end to end over the software-UART bridge (both
-	# under review — pureboot/autobaud.md): the host sends the 0xC0 calibration
-	# pulse, the loader times it, locks, and programs. Run on the near-flash 328P
+	# The autobaud loader driven end to end over the software-UART bridge:
+	# the host sends the 0xC0 calibration pulse, the loader times it, locks,
+	# and programs. Run on the near-flash 328P
 	# and the word-addressed 1284P — the two flash-addressing classes — and each
 	# at two clocks with the one binary, which is the clock-agnostic property
 	# autobaud exists for (test/pbautobaud.py). The fixture application banners

pureboot/CMakeLists.txt

@@ -136,6 +136,19 @@ elseif(LIBAVR_MCU STREQUAL "atmega644pa")
 	set(_pb_sim_mcu atmega644p)
 endif()
 
+# Where SRAM begins: the classic megas keep it right after the plain I/O
+# registers, the x8/x4 generations push it past their extended I/O file, and
+# the tinies match the classics. An autobaud loader's measured unit lives at
+# exactly this address (the host reads it there — pureboot.py), and the
+# unit-position test holds the layout to it.
+if(LIBAVR_MCU MATCHES "^atmega(8|16|32)a?$")
+	set(_pb_ram 0x60)
+elseif(LIBAVR_MCU MATCHES "^atmega")
+	set(_pb_ram 0x100)
+else()
+	set(_pb_ram 0x60)
+endif()
+
 # The function runs in its caller's scope, so everything it needs crosses
 # scopes as global properties.
 set_property(GLOBAL PROPERTY PUREBOOT_BASE_HEX ${_pb_base_hex})
@@ -155,6 +168,7 @@ set(PUREBOOT_SLOT ${_pb_slot} PARENT_SCOPE)
 set(PUREBOOT_LIMIT ${_pb_limit} PARENT_SCOPE)
 set(PUREBOOT_EEPROM ${_pb_eeprom} PARENT_SCOPE)
 set(PUREBOOT_DEFAULT_HZ ${_pb_hz} PARENT_SCOPE)
+set(PUREBOOT_RAM_START ${_pb_ram} PARENT_SCOPE)
 set(PUREBOOT_HAS_USART ${_pb_has_usart} PARENT_SCOPE)
 set(PUREBOOT_HAS_USART1 ${_pb_has_usart1} PARENT_SCOPE)
 set(PUREBOOT_SIM_MCU ${_pb_sim_mcu} PARENT_SCOPE)
@@ -212,7 +226,7 @@ endfunction()
 
 # pureboot_add_loader(<name> [CLOCK <hz>] [BAUD <bd>]
 #                     [SERIAL auto|hardware|software|autobaud] [USART <n>]
-#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>])
+#                     [RX <pin>] [TX <pin>] [TIMEOUT <s>] [OSCCAL <byte>])
 #
 # The loader target plus its flashable images (<name>.hex for a programmer,
 # <name>.bin for --update-loader). The resolved deployment is stamped on the
@@ -225,8 +239,15 @@ endfunction()
 # and one binary per chip serves every F_CPU and every rate. The stamped
 # PUREBOOT_HZ/PUREBOOT_BAUD then record what a harness should *drive* it at,
 # not what it was built for.
+#
+# OSCCAL bakes a measured oscillator trim into the loader (README.md: the
+# RC-oscillator deployment answer): the byte is written at the top of run(),
+# so every reset path — the watchdog hand-over included — runs on the
+# corrected clock. Orthogonal to the backend: an autobaud build may carry it
+# purely for the application's benefit, its own link being clock-free. No
+# value, no code.
 function(pureboot_add_loader name)
-	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT" "" ${ARGN})
+	cmake_parse_arguments(PB "" "CLOCK;BAUD;SERIAL;USART;RX;TX;TIMEOUT;OSCCAL" "" ${ARGN})
 	if(PB_UNPARSED_ARGUMENTS)
 		message(FATAL_ERROR "pureboot_add_loader(${name}): unknown arguments ${PB_UNPARSED_ARGUMENTS}")
 	endif()
@@ -325,6 +346,13 @@ function(pureboot_add_loader name)
 		set(_defines PUREBOOT_CLOCK_HZ=${PB_CLOCK} PUREBOOT_BAUD=${PB_BAUD} PUREBOOT_TIMEOUT=${PB_TIMEOUT}
 		    ${_serial_defines})
 	endif()
+	if(DEFINED PB_OSCCAL)
+		math(EXPR _osccal "${PB_OSCCAL}" OUTPUT_FORMAT DECIMAL)
+		if(_osccal LESS 0 OR _osccal GREATER 255)
+			message(FATAL_ERROR "pureboot_add_loader(${name}): OSCCAL ${PB_OSCCAL} is not one byte")
+		endif()
+		list(APPEND _defines PUREBOOT_OSCCAL=${_osccal})
+	endif()
 
 	add_executable(${name} ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/pureboot.cpp)
 	target_link_libraries(${name} PRIVATE libavr)

pureboot/README.md

@@ -48,12 +48,14 @@ it carries the calibration machinery and no clock at all.
 is 510 bytes, since the slot's last word is the trampoline.
 
 The tightest fit in the whole space is the 1284s' autobaud build deployed on a
-USART's own pins, 506 of its 512 — they alone carry the far-flash machinery
-(ELPM reads, RAMPZ page commands), autobaud alone carries the calibration loop,
-and a bit-banged link on a USART's pins alone has to release it (below). The
-same build on the default pins is 502. The flash bank riding in a transfer's
-selector byte keeps even those chips' addressing the same 16-bit form every
-other chip uses, which is why they are no longer the outlier they were.
+USART's own pins with the `OSCCAL` trim baked, 510 of its 512 — they alone
+carry the far-flash machinery (ELPM reads, RAMPZ page commands), autobaud
+alone carries the calibration loop, a bit-banged link on a USART's pins alone
+has to release it (below), and the trim adds its one register write. Without
+the trim that build is 504; on the default pins, 502. The flash bank riding
+in a transfer's selector byte keeps even those chips' addressing the same
+16-bit form every other chip uses, which is why they are no longer the
+outlier they were.
 
 The software UART enables the RX pull-up; TX idles high. All multi-byte wire
 quantities are little-endian.
@@ -72,6 +74,7 @@ repo's build and by a downstream project alike:
 | `USART <n>` | the USART instance (x4 megas carry two) | 0 |
 | `RX <pin>`, `TX <pin>` | software-UART pins | `pb0`, `pb1` |
 | `TIMEOUT <s>` | the activation window | 8 |
+| `OSCCAL <byte>` | a measured oscillator trim, applied before anything runs | none — no value, no code |
 
 The default baud is the fastest of 115200/57600/38400/19200/9600 the clock
 reaches within 2.5 % — the same U2X-included divisor search libavr's baud
@@ -122,12 +125,13 @@ window per reset. Measure with an application in place.
 A downstream project brings its usual libavr setup (the `libavr` target, the
 chip via the `LIBAVR_MCU` toolchain preset), consumes this directory, and
 states its deployment — an ATmega328P on its shipped 1 MHz fuses with the
-software UART on hand-picked pins, say:
+software UART on hand-picked pins, say. A submodule pins the loader version
+(the tags name them; this repo pins its own libavr the same way), where
+FetchContent tracks whatever `main` is:
 
 ```cmake
-FetchContent_Declare(bootloader GIT_REPOSITORY git@git.blackmark.me:avr/bootloader.git GIT_TAG main)
-FetchContent_MakeAvailable(bootloader)
-add_subdirectory(${bootloader_SOURCE_DIR}/pureboot pureboot)
+# git submodule add <forge>/avr/bootloader.git bootloader   — or FetchContent
+add_subdirectory(bootloader/pureboot pureboot)
 
 pureboot_add_loader(myboot CLOCK 1000000 SERIAL software TX pb1 RX pb5)
 ```
@@ -285,7 +289,9 @@ Two generations exist. **1 through 4** speak one session — a 12-byte info bloc
 from `b`, and a command per memory (`R`/`W` flash, `r`/`w` EEPROM, `F` fuses).
 **5** replaced those with the single `G`/`g` pair over selector-named spaces
 above; the shipped tool speaks both, choosing on the version it reads, so a
-deployed pureboot 4 stays drivable and self-updatable to 5.
+deployed pureboot 4 stays drivable and self-updatable to 5. **6** changes
+nothing on the wire: it marks the builds that may carry a baked `OSCCAL` trim
+(Configuration), so a tool driving an update knows such images exist.
 
 Every closed generation is tagged in this repo at its era's last commit — the
 commit just before the next version bump, so a tag holds everything its
@@ -350,6 +356,18 @@ mega (SPM only executes from the boot section — reflash the .hex), but *runs*
 on a patched-vector chip, and the ordinary `--update-loader` flow re-homes it
 into the top slot from there (`pureboot.rehome`).
 
+**Fixed-baud on an internal RC oscillator is a deployment risk the build
+cannot see.** The factory trim is ±10 % where an 8N1 frame survives about
+±4: a part at the edge answers nothing at the built rate, and the symptom —
+silence — reads as a wiring fault (a real ATtiny13A measured −5.5 %, outside
+every standard rate at its own documented default). The **autobaud build is
+the deployment-proof backend**: it has no rate to miss. Where fixed-baud on
+RC is wanted anyway, measure first and bake the trim: an autobaud session's
+`--info` prints the part's true clock from the loader's own measured bit
+period, OSCCAL moves the oscillator about 1 % per step, and `OSCCAL <byte>`
+builds the correction in — one build–measure iteration converges. A loader
+already deployed and silent is diagnosed with `--scan` (Host tool).
+
 ## Updating the loader
 
 `pureboot.py --update-loader new_pureboot.bin` replaces the resident loader
@@ -374,6 +392,11 @@ The host retunes on the open port, so no DTR pulse resets the copy it is talking
 to. Omit them against a changed link and the update stops after installing the
 staging copy, saying so and naming this as the cause.
 
+An `OSCCAL`-baked image is a link change in effect even at an unchanged rate
+on paper: the staging copy shifts the physical clock the moment its `run()`
+starts, and from then on speaks exactly what it was built for. Declare it
+like any other link change — `--staged-baud` with the new build's rate.
+
 The preflight refuses an image built for another chip: the stamp every pureboot
 binary carries must resolve to the device's own geometry, and the error names
 both. Die revisions share their base signature and geometry, so their images
@@ -430,7 +453,19 @@ application data into a mega's reset walk region.
 
 `--autobaud` opens with the calibration pulse instead of the plain knock, for a
 loader built `SERIAL autobaud`; the rest of the session is identical, at
-whatever `--baud` the host chose.
+whatever `--baud` the host chose. Its `--info` adds the **measured clock** —
+the loader's bit-period unit, decoded and multiplied by the session rate —
+which is the number an `OSCCAL` bake or a fixed-baud build for the part is
+held against; `--clock <hz>` states the drift against a nominal.
+
+`--scan` is the diagnosis once a fixed-baud loader has gone silent: it walks
+±10 % around `--baud` in 2 % steps, nearest first, one probe per activation
+window — reset the target as each probe announces itself (a board with DTR
+wired to reset is pulsed by the probe's own port-open). A loader
+off-frequency answers at its oscillator's ratio, and the report gives the
+found rate as the session workaround, the offset, the OSCCAL correction's
+direction at ~1 % per step, and the autobaud way out. Standalone — no other
+operation combines with it.
 
 `--peek ADDR[:N]` and `--poke ADDR:HEX` reach the data space (pureboot 5) —
 SRAM, and through the same address space the register file and every I/O
@@ -480,7 +515,12 @@ Per chip preset, `ctest` runs:
   too. The timeout is a constant and is no axis;
 - `pureboot_autobaud.size` — the clock-free build, which has no clock or baud
   axis of its own: one binary per chip has to serve every point the matrix
-  below sweeps;
+  below sweeps. `pureboot*osccal*.size` add the `OSCCAL` trim on the stock
+  shape and on the tightest image in the space (autobaud on a USART's own
+  pins), holding both of the trim write's addressing encodings to the budget;
+- `pureboot_autobaud.unit` — the measured bit period is the loader's only RAM
+  object and sits exactly at ram_start, where `--info` reads it: wire
+  contract, not layout accident;
 - `pbm_*.size` — with `PUREBOOT_FULL_MATRIX=1`, the exhaustive cross product
   replacing that compact matrix, on **every** chip: every plausible oscillator
   (the internal ones, the CKDIV8 floor, the plain and the UART crystals) ×
@@ -503,6 +543,13 @@ Per chip preset, `ctest` runs:
   recovery properties, the surgery, the staging composition, the boot-fuse
   decode, the update preflight over synthetic fuse bytes, and the repairing
   verify against a fake device;
+- `pureboot.scan` — `--scan`'s walk and report logic: the probe order, the
+  rate arithmetic, and the trim advice's direction. A pty carries bytes at
+  any termios rate, so the rate physics itself belongs to the hardware
+  harness, and what the wire would arbitrate is pinned as logic;
+- `presets.generated` — CMakePresets.json matches its generator
+  (`tools/make_presets.py --check`), so a hand edit or a generator change
+  cannot drift the pair apart;
 - `pureboot.protocol` — end to end against a simavr device
   (`test/pureboot_device.c`: a hardware USART as a pty, or a cycle-timed
   GPIO⇄pty bridge for a software-UART build, plus the SPM/NVM module simavr's
@@ -535,15 +582,21 @@ Per chip preset, `ctest` runs:
 - `pureboot.update` — the full `--update-loader` flow, then every power-fail
   phase: the device is killed mid-write, restarted from its flash dump, and a
   re-run must complete the update with the application intact;
+- `pureboot.osccal` (328P, t85) — a loader built with the `OSCCAL` axis holds
+  the trim register at the built byte from its first prompt, observed through
+  the wire on one chip per addressing encoding (`sts` and low-I/O `out`);
 - `pureboot.autobaud` (328P, 1284P) — the clock-free build over the GPIO⇄pty
   bridge: the calibration handshake, a flash + EEPROM + fuse round trip against
   the simulator's own memory, a data-space round trip, the hand-over — then the
   same binary again at double the clock, which is the property the backend
-  exists for. A lone calibration pulse with no knock behind it must still let
+  exists for. The measured clock `--info` prints is asserted against the
+  simulator's exact clock, inside the unit encoding's own envelope, at both
+  points. A lone calibration pulse with no knock behind it must still let
   the application boot, so no wait in activation can be unbounded.
 
-`size`, `pi`, `planner` and `handshake` are host logic and run anywhere; the
-simulator-driven targets need simavr and a pty, so they are POSIX-only.
+`size`, `unit`, `pi`, `planner`, `scan` and `handshake` are host logic and run
+anywhere; the simulator-driven targets need simavr and a pty, so they are
+POSIX-only.
 
 ## Hardware
 

pureboot/pureboot.cpp

@@ -72,9 +72,17 @@ constexpr std::uint8_t timeout_seconds = PUREBOOT_TIMEOUT;
 #endif
 constexpr avr::uint24_t autobaud_budget = PUREBOOT_AUTOBAUD_POLLS;
 
+// A build may bake a measured oscillator trim (README.md: the RC-oscillator
+// deployment answer); the byte is applied at the top of run(). Orthogonal to
+// the serial backend — an autobaud build may carry it for the application's
+// benefit alone.
+#if defined(PUREBOOT_OSCCAL)
+static_assert(PUREBOOT_OSCCAL >= 0 && PUREBOOT_OSCCAL <= 0xff, "PUREBOOT_OSCCAL is one OSCCAL byte");
+#endif
+
 // The loader's one identity number. The protocol carries none of its own —
 // a version implies it, and the host tool holds that map (README.md).
-constexpr std::uint8_t version = 5;
+constexpr std::uint8_t version = 6;
 
 // The image's identity stamp, for the host tool rather than for the wire: an
 // update image is a bare 512-byte slot, and without this nothing in it says
@@ -477,6 +485,12 @@ void fill_page(std::uint8_t bank, std::uint16_t at)
 
 [[noreturn]] void run()
 {
+#if defined(PUREBOOT_OSCCAL)
+	// The build's oscillator trim, ahead of everything — the WDRF bail
+	// included — so every path out of reset, the watchdog hand-over to the
+	// application first among them, runs on the corrected clock.
+	avr::clock::calibrate(PUREBOOT_OSCCAL);
+#endif
 	// A watchdog reset belongs to the application, whose watchdog stays forced
 	// on until it clears WDRF — no activation window in its way.
 	if (avr::hw::field_impl<wdrf_field()>::test())

pureboot/pureboot.py

@@ -20,17 +20,21 @@ if os.name == "nt":
     import ctypes
     from ctypes import wintypes
 else:
+    import array
+    import fcntl
     import select
     import termios
 
 PROMPT = b"+"
-VERSION = 5  # this tool's own version — free to drift from a loader's
-# The loader versions this tool speaks. A pureboot version implies its wire
+VERSION = 6  # this tool's own version — free to drift from a loader's
+# The loader versions this tool can drive. A pureboot version implies its wire
 # protocol, which carries no number of its own, so this window is where that
-# map lives: every version so far speaks the same protocol, and one that
-# changes it becomes the new floor here.
+# map lives: the tool keeps a decoder for every generation in it (1–4 speak
+# the per-memory commands, 5 the unified pair; 6 marks the OSCCAL-carrying
+# builds and changes nothing on the wire), and a version it has no decoder
+# for moves the floor.
 OLDEST_LOADER = 1
-NEWEST_LOADER = 5
+NEWEST_LOADER = 6
 SLOT = 512  # the loader slot, on every chip
 RETRIES = 3  # rewrites of a page that reads back wrong, before the run stops
 
@@ -41,6 +45,13 @@ RETRIES = 3  # rewrites of a page that reads back wrong, before the run stops
 UNIFIED_LOADER = 5
 SP_FLASH, SP_EEPROM, SP_RAM, SP_FUSE, SP_SPM = 0, 1, 2, 3, 4
 
+# A v5+ autobaud loader keeps its measured bit period at ram_start, encoded
+# as delay-loop counts: (bit cycles − UNIT_DISCOUNT) / UNIT_LOOP_CYCLES,
+# floored — the spin granule and per-bit overhead of libavr's software UART.
+# --info undoes the encoding to report the true clock, which therefore sits
+# within one granule below it.
+UNIT_LOOP_CYCLES, UNIT_DISCOUNT = 4, 8
+
 # A selector's high nibble is the flash bank — the address bits above the 16-bit
 # wire address — so a transfer names a byte address within one 64 KiB bank and
 # no command has to speak word addresses. No single transfer may cross a bank
@@ -66,31 +77,36 @@ CALIBRATE = 0xC0
 # from its chip database at build time). Die revisions that share a signature
 # share this row, as they share the silicon.
 CHIP_GEOMETRY = {
-    #  signature  : (flash, page, eeprom, patch_vector)
-    (0x1E, 0x90, 0x07): (1024, 32, 64, True),      # ATtiny13/13A
-    (0x1E, 0x91, 0x08): (2048, 32, 128, True),     # ATtiny25
-    (0x1E, 0x92, 0x06): (4096, 64, 256, True),     # ATtiny45
-    (0x1E, 0x93, 0x0B): (8192, 64, 512, True),     # ATtiny85
-    (0x1E, 0x92, 0x05): (4096, 64, 256, True),     # ATmega48/48A
-    (0x1E, 0x92, 0x0A): (4096, 64, 256, True),     # ATmega48P/48PA
-    (0x1E, 0x93, 0x07): (8192, 64, 512, False),    # ATmega8/8A
-    (0x1E, 0x93, 0x0A): (8192, 64, 512, False),    # ATmega88/88A
-    (0x1E, 0x93, 0x0F): (8192, 64, 512, False),    # ATmega88P/88PA
-    (0x1E, 0x94, 0x03): (16384, 128, 512, False),  # ATmega16/16A
-    (0x1E, 0x94, 0x06): (16384, 128, 512, False),  # ATmega168/168A
-    (0x1E, 0x94, 0x0B): (16384, 128, 512, False),  # ATmega168P/168PA
-    (0x1E, 0x94, 0x0A): (16384, 128, 512, False),  # ATmega164P/164PA
-    (0x1E, 0x94, 0x0F): (16384, 128, 512, False),  # ATmega164A
-    (0x1E, 0x95, 0x02): (32768, 128, 1024, False), # ATmega32/32A
-    (0x1E, 0x95, 0x0F): (32768, 128, 1024, False), # ATmega328P
-    (0x1E, 0x95, 0x14): (32768, 128, 1024, False), # ATmega328
-    (0x1E, 0x95, 0x08): (32768, 128, 1024, False), # ATmega324P
-    (0x1E, 0x95, 0x11): (32768, 128, 1024, False), # ATmega324PA
-    (0x1E, 0x95, 0x15): (32768, 128, 1024, False), # ATmega324A
-    (0x1E, 0x96, 0x09): (65536, 256, 2048, False), # ATmega644/644A
-    (0x1E, 0x96, 0x0A): (65536, 256, 2048, False), # ATmega644P/644PA
-    (0x1E, 0x97, 0x05): (131072, 256, 4096, False),# ATmega1284P
-    (0x1E, 0x97, 0x06): (131072, 256, 4096, False),# ATmega1284
+    #  signature  : (flash, page, eeprom, patch_vector, ram_start)
+    # ram_start is where SRAM begins in data space: the classic megas and the
+    # tinies keep it right after the plain I/O registers (0x60), the x8/x4
+    # generations past their extended I/O file (0x100). An autobaud loader's
+    # measured bit period lives at exactly ram_start (its only RAM object;
+    # the loader's own build pins the layout), which is what --info reads.
+    (0x1E, 0x90, 0x07): (1024, 32, 64, True, 0x60),      # ATtiny13/13A
+    (0x1E, 0x91, 0x08): (2048, 32, 128, True, 0x60),     # ATtiny25
+    (0x1E, 0x92, 0x06): (4096, 64, 256, True, 0x60),     # ATtiny45
+    (0x1E, 0x93, 0x0B): (8192, 64, 512, True, 0x60),     # ATtiny85
+    (0x1E, 0x92, 0x05): (4096, 64, 256, True, 0x100),    # ATmega48/48A
+    (0x1E, 0x92, 0x0A): (4096, 64, 256, True, 0x100),    # ATmega48P/48PA
+    (0x1E, 0x93, 0x07): (8192, 64, 512, False, 0x60),    # ATmega8/8A
+    (0x1E, 0x93, 0x0A): (8192, 64, 512, False, 0x100),   # ATmega88/88A
+    (0x1E, 0x93, 0x0F): (8192, 64, 512, False, 0x100),   # ATmega88P/88PA
+    (0x1E, 0x94, 0x03): (16384, 128, 512, False, 0x60),  # ATmega16/16A
+    (0x1E, 0x94, 0x06): (16384, 128, 512, False, 0x100), # ATmega168/168A
+    (0x1E, 0x94, 0x0B): (16384, 128, 512, False, 0x100), # ATmega168P/168PA
+    (0x1E, 0x94, 0x0A): (16384, 128, 512, False, 0x100), # ATmega164P/164PA
+    (0x1E, 0x94, 0x0F): (16384, 128, 512, False, 0x100), # ATmega164A
+    (0x1E, 0x95, 0x02): (32768, 128, 1024, False, 0x60),  # ATmega32/32A
+    (0x1E, 0x95, 0x0F): (32768, 128, 1024, False, 0x100), # ATmega328P
+    (0x1E, 0x95, 0x14): (32768, 128, 1024, False, 0x100), # ATmega328
+    (0x1E, 0x95, 0x08): (32768, 128, 1024, False, 0x100), # ATmega324P
+    (0x1E, 0x95, 0x11): (32768, 128, 1024, False, 0x100), # ATmega324PA
+    (0x1E, 0x95, 0x15): (32768, 128, 1024, False, 0x100), # ATmega324A
+    (0x1E, 0x96, 0x09): (65536, 256, 2048, False, 0x100), # ATmega644/644A
+    (0x1E, 0x96, 0x0A): (65536, 256, 2048, False, 0x100), # ATmega644P/644PA
+    (0x1E, 0x97, 0x05): (131072, 256, 4096, False, 0x100),# ATmega1284P
+    (0x1E, 0x97, 0x06): (131072, 256, 4096, False, 0x100),# ATmega1284
 }
 
 VERBOSE = False
@@ -144,36 +160,60 @@ class Progress:
 
 
 class PosixPort:
-    """A raw serial port with deadline-based reads, over termios."""
+    """A raw serial port with deadline-based reads, over termios. A rate with
+    no B-constant — the off-nominal probes `--scan` walks — goes through
+    Linux's termios2 BOTHER; a platform without that ioctl refuses the rate
+    by name."""
+
+    # The termios2 ioctl pair and cflag bits, and the struct's ispeed/ospeed
+    # word offsets: four flag words, then a line-discipline byte and 19
+    # control chars padded to word 9 (include/uapi/asm-generic/termbits.h).
+    _TCGETS2, _TCSETS2 = 0x802C542A, 0x402C542B
+    _BOTHER, _CBAUD = 0o010000, 0o010017
+    _ISPEED, _OSPEED = 9, 10
 
     @staticmethod
     def _speed(baud):
+        return getattr(termios, f"B{baud}", None)
+
+    def _set_arbitrary(self, baud):
+        buf = array.array("i", [0] * (self._OSPEED + 1))
         try:
-            return getattr(termios, f"B{baud}")
-        except AttributeError:
-            raise Error(f"unsupported baud rate {baud}") from None
+            fcntl.ioctl(self.fd, self._TCGETS2, buf, True)
+            buf[2] = (buf[2] & ~self._CBAUD) | self._BOTHER
+            buf[self._ISPEED] = buf[self._OSPEED] = baud
+            fcntl.ioctl(self.fd, self._TCSETS2, buf)
+        except OSError:
+            raise Error(f"this platform cannot set {baud} Bd (no termios2)") from None
+
+    def _apply_baud(self, attrs, baud):
+        speed = self._speed(baud)
+        attrs[4] = attrs[5] = speed if speed is not None else termios.B38400
+        termios.tcsetattr(self.fd, termios.TCSANOW, attrs)
+        if speed is None:
+            self._set_arbitrary(baud)
+        self.baud = baud
 
     def __init__(self, path, baud):
         self.fd = os.open(path, os.O_RDWR | os.O_NOCTTY)
+        try:
             attrs = termios.tcgetattr(self.fd)
             attrs[0] = 0  # iflag
             attrs[1] = 0  # oflag
             attrs[2] = termios.CREAD | termios.CLOCAL | termios.CS8  # cflag
             attrs[3] = 0  # lflag
-        attrs[4] = attrs[5] = self._speed(baud)
             attrs[6][termios.VMIN] = 0
             attrs[6][termios.VTIME] = 0
-        termios.tcsetattr(self.fd, termios.TCSANOW, attrs)
-        self.baud = baud
+            self._apply_baud(attrs, baud)
+        except BaseException:
+            os.close(self.fd)
+            raise
 
     def set_baud(self, baud):
         """Retune the port without closing it — the fd stays open, so no DTR
         pulse and no reset. That matters: the only caller is mid-session with a
         loader copy that a reset would throw away."""
-        attrs = termios.tcgetattr(self.fd)
-        attrs[4] = attrs[5] = self._speed(baud)
-        termios.tcsetattr(self.fd, termios.TCSANOW, attrs)
-        self.baud = baud
+        self._apply_baud(termios.tcgetattr(self.fd), baud)
 
     def close(self):
         os.close(self.fd)
@@ -410,7 +450,7 @@ class Info:
         if geometry is None:
             sig = " ".join(f"{b:02x}" for b in signature)
             raise Error(f"unknown signature {sig} — this tool has no geometry for it")
-        flash, page, eeprom, patch = geometry
+        flash, page, eeprom, patch, _ = geometry
         base = flash - SLOT
         word_flash = flash > 0x10000
         wire_base = base // 2 if word_flash else base
@@ -446,6 +486,11 @@ class Info:
         # The hand-over target as 'J' takes it: the trampoline below the
         # loader, or word 0 where BOOTRST re-vectors reset in hardware.
         self.app_entry_word = (self.base - 2) // 2 if self.patch_vector else 0
+        # Where SRAM begins, from the signature — None only for a chip this
+        # tool has no geometry row for, which the wire-block path (v1–4)
+        # permits where from_identity refuses.
+        geometry = CHIP_GEOMETRY.get(tuple(self.signature))
+        self.ram = geometry[4] if geometry else None
 
     def describe(self):
         sig = " ".join(f"{b:02x}" for b in self.signature)
@@ -512,6 +557,7 @@ class Loader:
         drain absorbs whatever they produced."""
         deadline = time.monotonic() + wait
         knocks = 0
+        refusal = None
         while True:
             self.port.flush_input()
             self.port.write(knock)
@@ -533,12 +579,18 @@ class Loader:
                 except Error as failed:
                     if "pureboot" in str(failed):
                         raise
+                    # A malformed or unknown identity is retried as noise, but
+                    # it was an answer: if nothing better ever arrives, naming
+                    # it beats reporting silence.
+                    refusal = failed
                     self.info = None
                 if self.info is not None:
                     self._expect_prompt()
                     verbose(f"loader answered {what} {knocks}; identity read")
                     return self.info
             if time.monotonic() > deadline:
+                if refusal is not None:
+                    raise Error(f"no usable answer — the last identity reply failed: {refusal}")
                 raise Error("no answer — reset the device within its activation window")
 
     def connect(self, wait):
@@ -1351,6 +1403,62 @@ def op_fuses(loader):
     return fuse_bytes
 
 
+def scan_ratios():
+    """The probe walk, in percent of the built rate: the built rate itself
+    first, then ±10 % in 2 % steps nearest-first — a drifted oscillator near
+    its trim is the common case, and each probe costs a reset."""
+    return [0] + [sign * step for step in (2, 4, 6, 8, 10) for sign in (-1, 1)]
+
+
+def scan_rate(baud, pct):
+    return round(baud * (100 + pct) / 100)
+
+
+def scan_report(baud, pct, version, clock=None):
+    """The findings, one per line: the found rate is the session workaround,
+    its ratio to the built rate is the oscillator's offset, and the fixes are
+    the OSCCAL bake (≈1 %/step, opposing the drift) or the autobaud build."""
+    rate = scan_rate(baud, pct)
+    lines = [f"scan: answered at {rate} Bd ({pct:+d} % of the built rate) — pureboot {version}",
+             f"  session   --baud {rate}"]
+    if clock:
+        lines.append(f"  clock     ~{clock * (100 + pct) // 100} Hz (built for {clock})")
+    if pct:
+        direction = "lower" if pct > 0 else "higher"
+        lines.append(f"  fix       rebuild with OSCCAL ~{abs(pct)} steps {direction} (~1 %/step), "
+                     "or the autobaud build")
+    else:
+        lines.append("  fix       none — the built rate answers; check the earlier wiring instead")
+    return lines
+
+
+def op_scan(port_path, baud, wait, clock=None):
+    """A fixed-baud loader whose oscillator drifted still answers — at the
+    drifted ratio, since its rate scales with its clock. One probe per
+    activation window, and with an application resident the window opens
+    exactly once per reset, so each probe announces itself and expects a
+    fresh reset before knocking."""
+    for pct in scan_ratios():
+        rate = scan_rate(baud, pct)
+        print(f"scan: {rate} Bd ({pct:+d} %) — reset the target", flush=True)
+        try:
+            port = Port(port_path, rate)
+        except Error as unmakeable:
+            print(f"scan: {rate} Bd skipped — {unmakeable}")
+            continue
+        try:
+            info = Loader(port).connect(wait)
+        except Error:
+            continue
+        finally:
+            port.close()
+        for line in scan_report(baud, pct, info.version, clock):
+            print(line)
+        return
+    raise Error("no answer within ±10 % of the built rate — check the wiring, or deploy the "
+                "autobaud build, which has no rate to miss (README.md)")
+
+
 # -------------------------------------------------------------------- cli ---
 
 
@@ -1366,6 +1474,12 @@ def main():
     parser.add_argument("--autobaud", action="store_true",
                         help="drive an autobaud loader: send the 0xC0 calibration pulse and a single "
                              "knock, and take geometry from the signature (no clock/baud baked in)")
+    parser.add_argument("--scan", action="store_true",
+                        help="walk ±10%% around --baud for a fixed-baud loader gone silent — one "
+                             "reset per probe, standalone (README.md: deployment)")
+    parser.add_argument("--clock", type=int, metavar="HZ",
+                        help="the clock the loader was built for — lets --scan and an autobaud "
+                             "--info state drift in absolute terms")
     parser.add_argument("--info", action="store_true", help="print the device info block")
     parser.add_argument("--fuses", action="store_true", help="read the fuse and lock bytes")
     parser.add_argument("--update-loader", metavar="FILE", help="replace the loader with this pureboot binary")
@@ -1412,6 +1526,12 @@ def main():
         except (ValueError, AssertionError):
             parser.error("--assume-fuses takes 8 hex digits: low,lock,extended,high")
 
+    if args.scan:
+        if args.autobaud:
+            parser.error("--scan probes fixed rates; an autobaud loader has none to miss")
+        op_scan(args.port, args.baud, args.wait, args.clock)
+        return
+
     port = Port(args.port, args.baud)
     verbose(f"{args.port}: {args.baud} Bd 8N1, DTR/RTS asserted")
     try:
@@ -1421,6 +1541,18 @@ def main():
             print("device:")
             for line in info.lines():
                 print(f"  {line}")
+            if args.autobaud and info.ram is not None:
+                # The whole of the loader's RAM is the measured bit period at
+                # ram_start; decoded and times the rate this session drives,
+                # that is the true clock — the number to hold an OSCCAL bake
+                # or a fixed-baud build against (README.md: deployment). The
+                # autobaud identity path refuses unknown signatures, so ram is
+                # always known here; the guard states that dependency.
+                unit = int.from_bytes(loader.read_ram(info.ram, 2), "little")
+                cycles = unit * UNIT_LOOP_CYCLES + UNIT_DISCOUNT
+                clock = cycles * args.baud
+                offset = f", {(clock / args.clock - 1) * 100:+.1f} % of {args.clock}" if args.clock else ""
+                print(f"  measured     {clock} Hz ({cycles} cycles/bit × {args.baud} Bd{offset})")
         fuse_bytes = fuse_override
         if args.fuses or (args.update_loader and not info.patch_vector and fuse_bytes is None):
             read = op_fuses(loader)
@@ -1468,7 +1600,7 @@ def main():
 if __name__ == "__main__":
     try:
         main()
-    except Error as error:
+    except (Error, OSError) as error:
         print(f"error: {error}", file=sys.stderr)
         sys.exit(1)
     except KeyboardInterrupt:

test/check_unit.cmake

@@ -0,0 +1,23 @@
+# Asserts the autobaud loader's measured unit is the first RAM object: the
+# host tool reads the bit period from ram_start (--info's measured clock), so
+# the unit's address is wire contract. Run as
+#   cmake -DOBJDUMP=... -DELF=... -DRAM_START=<data address> -P check_unit.cmake
+
+execute_process(COMMAND ${OBJDUMP} -t ${ELF} OUTPUT_VARIABLE _syms RESULT_VARIABLE _res)
+if(NOT _res EQUAL 0)
+	message(FATAL_ERROR "${OBJDUMP} -t ${ELF} failed")
+endif()
+
+# The symbol line: "00800100 l     O .noinit  00000002 <mangled>unit_E".
+string(REGEX MATCH "\n0*([0-9a-f]+)[^\n]+[ \t][^ \t\n]*unit_[^ \t\n]*\n" _line "${_syms}")
+if(NOT _line)
+	message(FATAL_ERROR "no unit_ symbol in ${ELF} — is this the autobaud loader?")
+endif()
+
+# AVR data-space symbols carry the 0x800000 VMA offset.
+math(EXPR _want "0x800000 + ${RAM_START}" OUTPUT_FORMAT HEXADECIMAL)
+math(EXPR _have "0x${CMAKE_MATCH_1}" OUTPUT_FORMAT HEXADECIMAL)
+if(NOT _have STREQUAL _want)
+	message(FATAL_ERROR "unit_ sits at ${_have}, ram_start is ${_want} — the host peeks ram_start")
+endif()
+message(STATUS "unit_ at ${_have} == ram_start")

test/pbautobaud.py

@@ -16,6 +16,7 @@ baked in.
 """
 
 import os
+import re
 import sys
 import time
 
@@ -58,11 +59,23 @@ def main():
         try:
             # The host tool, in autobaud mode, sends the 0xC0 calibration pulse
             # and a single knock at `baud`; the loader locks to it.
-            out = pbsim.run_tool(tool, device.pty, baud, "--autobaud", "--info", "--fuses",
-                                 "--flash", app_bin, "--eeprom", ee_path, "--stay")
+            out = pbsim.run_tool(tool, device.pty, baud, "--autobaud", "--info", "--clock", str(hz),
+                                 "--fuses", "--flash", app_bin, "--eeprom", ee_path, "--stay")
             for needed in ("version", "signature", "fuses", "verify:", "stays"):
                 if needed not in out:
                     fail(f"{label}: session output lacks {needed!r}\n{out}")
+            # The measured clock, decoded from the unit at ram_start. The
+            # runner's clock is exact, so the figure must land inside the
+            # encoding's own envelope: the loader floors the bit period to
+            # 4-cycle spin granules after an 8-cycle discount, and the edge
+            # poll can shave a few cycles more — one granule of slack below
+            # the true clock, none above (in cycles per bit, times the rate).
+            measured = re.search(r"measured\s+(\d+) Hz", out)
+            if not measured:
+                fail(f"{label}: --info lacks the measured clock\n{out}")
+            measured = int(measured.group(1))
+            if not hz - 19 * baud <= measured <= hz + 4 * baud:
+                fail(f"{label}: measured clock {measured} Hz is {measured - hz:+d} off the true {hz}")
             # Read both memories back over the locked link and check them.
             read_flash = os.path.join(workdir, f"rf_{label}.bin")
             read_eeprom = os.path.join(workdir, f"re_{label}.bin")

test/pbosccal.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""The build-time OSCCAL trim, observed through the wire: a loader built with
+the OSCCAL axis holds the trim register at the built byte from its first
+prompt on — the write sits at the top of run(), ahead of the WDRF bail, so
+every path out of reset runs on the corrected clock. simavr's clock does not
+follow OSCCAL, which is what makes the value assertable at all: the register
+is plain state there, and the peek must return exactly what the build
+declared rather than whatever the oscillator needed.
+
+Usage: pbosccal.py <device_bin> <pureboot_elf> <mcu> <hz> <base_hex> <page>
+                   <baud> <osccal_addr> <osccal_value> <tool_py> <workdir>
+                   [link]
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    args = sys.argv[1:]
+    link = args.pop() if len(args) == 12 else None
+    (device_bin, elf, mcu, hz, base_hex, page, baud, addr, value, tool, workdir) = args
+    addr, value, baud = int(addr, 0), int(value, 0), int(baud)
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    import pbsim
+
+    os.makedirs(workdir, exist_ok=True)
+    dump = os.path.join(workdir, "flash_dump.bin")
+    device = pbsim.Device(device_bin, elf, mcu, hz, base_hex, page, baud, dump, link=link)
+    try:
+        out = pbsim.run_tool(tool, device.pty, baud, "--peek", f"{addr:#x}:1")
+        want = f"{addr:#06x}  {value:02x}"
+        if want not in out:
+            fail(f"OSCCAL at {addr:#x} did not read back {value:#04x}:\n{out}")
+    finally:
+        device.stop()
+    print("OK")
+
+
+if __name__ == "__main__":
+    main()

test/pureboot_device.c

@@ -193,10 +193,11 @@ static avr_cycle_count_t tx_sample(avr_t *mcu, avr_cycle_count_t when, void *par
 		tx_shift = (uint8_t)((tx_shift >> 1) | (tx_level ? 0x80 : 0));
 		if (++tx_bit < 8)
 			return when + bit_cycles;
-		/* The byte is not delivered until its stop bit has passed. A real
-		 * receiver cannot answer sooner, and a host that did would put its
-		 * start bit on the wire while the device is still driving the stop
-		 * bit — which the device, transmitting, is not watching for. */
+		/* The byte is delivered at the stop bit's sampling point (9.5 bit
+		 * times), where a hardware receiver raises its RXC — not sooner: a
+		 * host answering before the stop bit would put its start bit on the
+		 * wire while the device is still driving, which the device,
+		 * transmitting, is not watching for. */
 		return when + bit_cycles;
 	}
 	if (write(pty_master, &tx_shift, 1) != 1)
@@ -412,6 +413,14 @@ int main(int argc, char *argv[])
 			fprintf(stderr, "device: cannot read %s\n", argv[1]);
 			return 1;
 		}
+		// An image past flash end would smash the simulator's heap and turn
+		// into phantom peripheral behavior (lessons: believe the size gate
+		// first) — refuse it loudly instead.
+		if (base + fw.flashsize > avr->flashend + 1) {
+			fprintf(stderr, "device: %u B at 0x%x runs past flash end 0x%x — image does not fit its slot\n",
+			        (unsigned)fw.flashsize, base, avr->flashend);
+			return 1;
+		}
 		memcpy(avr->flash + base, fw.flash, fw.flashsize);
 	}
 	// The boot-sectioned megas enter the loader in hardware (BOOTRST, not

test/test_scan.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""--scan's walk and report logic, no simulator: the probe order, the rate
+arithmetic, and the advice's direction. The rate physics itself is not
+sim-testable — a pty carries bytes at any termios rate — so what the wire
+would arbitrate is pinned here as logic instead.
+
+Usage: test_scan.py <tool_py>
+"""
+
+import os
+import sys
+
+
+def fail(message):
+    print(f"FAIL: {message}")
+    sys.exit(1)
+
+
+def main():
+    sys.path.insert(0, os.path.dirname(os.path.abspath(sys.argv[1])))
+    import pureboot as pb
+
+    walk = pb.scan_ratios()
+    if walk != [0, -2, 2, -4, 4, -6, 6, -8, 8, -10, 10]:
+        fail(f"probe walk is not built-rate-first, nearest-out: {walk}")
+
+    if pb.scan_rate(9600, 4) != 9984 or pb.scan_rate(9600, -4) != 9216:
+        fail("probe rate arithmetic")
+    if pb.scan_rate(115200, 0) != 115200:
+        fail("the built rate must probe unchanged")
+
+    # A loader answering fast means a fast oscillator: the trim goes down.
+    report = "\n".join(pb.scan_report(9600, 4, 6))
+    for needle in ("9984", "+4 %", "--baud 9984", "4 steps lower", "pureboot 6"):
+        if needle not in report:
+            fail(f"+4 % report lacks {needle!r}:\n{report}")
+    report = "\n".join(pb.scan_report(9600, -6, 6))
+    if "6 steps higher" not in report:
+        fail(f"-6 % report advises the wrong direction:\n{report}")
+
+    report = "\n".join(pb.scan_report(9600, 0, 6))
+    if "none" not in report or "steps" in report:
+        fail(f"an on-rate answer must advise no trim:\n{report}")
+
+    report = "\n".join(pb.scan_report(9600, 4, 6, clock=9600000))
+    if "9984000" not in report:
+        fail(f"the absolute clock must scale with the found ratio:\n{report}")
+
+    # The walk's rates mostly have no termios B-constant, so the POSIX port
+    # must set them through termios2 — probed on a pty, which accepts the
+    # ioctl without caring about the speed. Without this every off-nominal
+    # probe would abort the walk on the platform --scan matters most on.
+    if os.name == "posix":
+        import pty
+
+        master, slave = pty.openpty()
+        try:
+            port = pb.Port(os.ttyname(slave), pb.scan_rate(9600, 4))
+            port.set_baud(pb.scan_rate(9600, -4))
+            port.close()
+        except pb.Error as error:
+            fail(f"PosixPort refused an off-nominal probe rate: {error}")
+        finally:
+            os.close(master)
+            os.close(slave)
+
+    print("OK")
+
+
+if __name__ == "__main__":
+    main()

tools/make_presets.py

@@ -7,11 +7,14 @@ port's TUs compile identically; the sims prove nothing new there) exist for
 libavr's reflect spot set only, mirroring its rule: the full reflect matrix
 is never built, one chip per hardware class and pack vintage is.
 
-Run from the repo root: tools/make_presets.py
+Run from the repo root: tools/make_presets.py — or with --check, which
+verifies the committed file matches this generator and edits nothing (the
+ctest entry `presets.generated` runs that, so drift reds the gate).
 """
 
 import json
 import os
+import sys
 
 CHIPS = [
     "attiny13", "attiny13a", "attiny25", "attiny45", "attiny85",
@@ -41,7 +44,7 @@ def main():
         "hidden": True,
         "generator": "Ninja",
         "binaryDir": "${sourceDir}/build/${presetName}",
-        "toolchainFile": "$env{LIBAVR_ROOT}/cmake/avr-toolchain.cmake",
+        "toolchainFile": "${sourceDir}/libavr/cmake/avr-toolchain.cmake",
         "cacheVariables": {
             "CMAKE_BUILD_TYPE": "Release",
             "CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
@@ -72,6 +75,9 @@ def main():
     for chip in REFLECT_SPOT:
         add(chip, "reflect")
 
+    # CMake rejects unknown fields in the presets root, $comment included, so
+    # the file cannot carry a generated-file marker; the --check ctest is the
+    # whole of rule 10's guard here.
     presets = {
         "version": 8,
         "configurePresets": configure,
@@ -79,12 +85,19 @@ def main():
         "testPresets": test,
         "workflowPresets": workflows,
     }
+    rendered = json.dumps(presets, indent=1) + "\n"
     path = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "CMakePresets.json")
+    if "--check" in sys.argv[1:]:
+        current = open(path).read() if os.path.exists(path) else ""
+        if current != rendered:
+            print("CMakePresets.json does not match its generator — run tools/make_presets.py")
+            return 1
+        return 0
     with open(path, "w") as f:
-        json.dump(presets, f, indent=1)
-        f.write("\n")
+        f.write(rendered)
     print(f"{len(CHIPS)} chips, {len(REFLECT_SPOT)} reflect: {os.path.normpath(path)}")
+    return 0
 
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main())

tools/pbhw.py

@@ -72,6 +72,39 @@ class Suite:
             except Exception:
                 pass
 
+    def scan(self) -> None:
+        """The --scan walk against real termios and a real oscillator: every
+        probe rate must open a port (the off-nominal rates exist only through
+        termios2), and one probe must answer — the nominal on a healthy board,
+        a neighbor on a drifted one. The rig injects the one reset per probe
+        the operator supplies in the field; this is the rate physics the
+        simulator cannot arbitrate (a pty carries bytes at any rate), pinned
+        on silicon."""
+        module = pbrig.load_pureboot(self.rig.d.pureboot)
+        found = None
+        try:
+            for pct in module.scan_ratios():
+                rate = module.scan_rate(self.rig.d.baud, pct)
+                self.rig.reset()
+                try:
+                    port = module.Port(self.rig.d.port, rate)
+                except module.Error as error:
+                    self.check("scan opens every probe rate", False, f"{rate} Bd: {error}")
+                    return
+                try:
+                    module.Loader(port).connect(min(self.rig.d.wait, 6.0))
+                    found = pct
+                    break
+                except module.Error:
+                    continue
+                finally:
+                    port.close()
+        except Exception as error:  # noqa: BLE001 — a rig hiccup is a result
+            self.check("scan walks the probe ladder", False, str(error)[:70])
+            return
+        self.check("scan finds the board's rate", found is not None,
+                   "no probe answered" if found is None else f"{found:+d} % of {self.rig.d.baud} Bd")
+
     def eeprom(self, info) -> None:
         size = info.eeprom_size
         if not size:
@@ -161,6 +194,10 @@ class Suite:
             print("\nthe loader never answered; nothing below can be trusted")
             return 1
 
+        if not self.rig.d.autobaud:
+            print("\nscan")
+            self.scan()
+
         print("\nEEPROM")
         self.eeprom(info)