bootloader/pureboot

pureboot

A serial bootloader on libavr, pure by constraint: one C++ source, no inline assembly, no global register variables (attributes allowed), built for every chip libavr targets, 512 bytes on each — 488 B on the ATtiny13A, 502 B on the ATtiny85, 504 B on the ATmega328P. The device speaks primitives; every composite — verify, erase, reset-vector surgery, updating the loader itself — lives in the host tool (pureboot.py).

The image is position-independent: control flow is PC-relative, the read/write paths take wire addresses, the write guard protects the 512-byte slot the code is running in (from the runtime return address), the info block is addressed from that same anchor, and the application jump is an indirect call to an absolute entry. The identical binary therefore runs from any 512-byte slot with every command intact — which makes pureboot its own staging loader: the host installs the same binary one slot below the resident, jumps into it, and lets it rewrite the resident. On the tinies the budget is 510, not 512: a slot's last word belongs to the host-managed trampoline (below).

Link

Chip	Serial	Baud	Clock assumed
ATmega328P	USART0, RXD/TXD = PD0/PD1	115200 8N1	16 MHz crystal
ATtiny85	software UART, RX = PB0, TX = PB1	57600 8N1	8 MHz internal RC
ATtiny13A	software UART, RX = PB0, TX = PB1	57600 8N1	9.6 MHz internal RC

The tiny RX pin has its pull-up enabled; TX idles high. All multi-byte quantities on the wire are little-endian.

Activation

Reset enters the loader (BOOTRST on the mega, the patched reset vector on the tinies) — except a watchdog reset, which hands straight to the application (the application owns its watchdog; it must clear WDRF itself, which also releases the WDRF-forced WDE).

The host then has one activation window per awaited byte to knock: p then b. Each awaited byte gets a fresh window; any other byte is discarded and awaited again (line noise cannot lock the loader, only delay it). A window expiring with an idle line boots the application.

The window length is a compile-time constant — 8 s by default, another value via the PUREBOOT_TIMEOUT CMake cache variable — so the whole EEPROM belongs to the application; pureboot never uses it for its own state. Re-timing a deployed loader is a self-update with a re-timed build (below).

Session

After the knock the loader stays in its command loop until J jumps away or the chip resets. Before reading each command it waits for any pending EEPROM write to finish and sends the prompt + (0x2b) — the prompt is therefore also the completion ack of the previous command. A session is: await +, send a command, read its reply, repeat.

Cmd	Arguments	Reply
b	—	the 12-byte info block
R	addr16, n8	n flash bytes (n = 0 means 256)
W	addr16, then one page of data	— (completion = next prompt)
r	addr16, n8	n EEPROM bytes (n = 0 means 256)
w	addr16, n8, then n data bytes	+ per byte, sent once its write has begun
F	—	4 bytes: low fuse, lock, extended fuse, high fuse
J	word address (16-bit)	+, then execution continues there
other	—	ignored; the loop re-prompts (send a junk byte, await +, to resync)

W streams exactly one SPM page (size from the info block) into the buffer, then erases and programs; the address must be page-aligned. Pages inside the 512-byte slot the loader is running in are drained but never programmed — a broken host cannot brick the running copy, and a staged copy may rewrite the resident slot. w is host-paced: send the next byte only after the previous byte's +. F returns the bytes in the hardware's Z order; on a chip without an extended fuse byte (the ATtiny13A) that slot carries no meaning. Fuse writing does not exist: SPM reaches flash (and, on the mega, lock bits) only — fuse bytes are external-programming territory by hardware.

J is the one control-transfer primitive: the host uses it to run the application (word 0 on the mega, the trampoline word on the tinies — both known from the info block) and to move between loader copies during a self-update. A jump to a loader slot's base re-enters that copy's own startup; it must then be knocked afresh.

The info block (b):

Offset	Content
0–2	'P', 'B', protocol version (1)
3–5	device signature
6	SPM page size in bytes
7–8	loader base — application flash ends here
9–10	EEPROM size
11	bit 0 set: host must patch the reset vector (no hardware boot section)

Composites are the host's job: verify = read back and compare, erase = write 0xff (per page for flash, per byte for EEPROM).

Deployment

ATmega328P: program the loader at 0x7e00 with an external programmer. Two fuse profiles, same binary:

BOOTSZ	BOOTRST	Behavior
256 words (512 B)	programmed	Standalone: reset always enters the loader; self-update impossible (the staging slot lies outside the boot section, where SPM is disabled).
512 words (1 KB)	unprogrammed	Self-update, app-first: reset always boots the application, which owns all 31.5 KB and must offer its own jump to 0x7e00 to reach the loader (a virgin chip reaches it by reset across erased flash). Updates are power-fail-safe except mid-rewrite of the resident slot itself (no reset path leads to the staging copy then).
512 words (1 KB)	programmed	Self-update, loader-first: reset lands at 0x7c00 — the staging slot, normally erased, so execution walks up into the loader; during an update it is the staging copy itself, so a mid-rewrite power loss recovers by reset. The loss windows move to the staging install/retire page writes instead (page-write scale). The host keeps [0x7c00, 0x7e00) clear of application data (--force overrides).

Applications are flashed unmodified — word 0 stays the application's own reset vector, and the hand-over jumps to 0.

Tinies (no boot section): program the loader at flash − 512; erased flash below it walks up into the loader, so a virgin chip activates. When flashing an application the host performs reset-vector surgery: word 0 is rewritten to rjmp to the loader base, and the application's own entry is re-encoded as a trampoline rjmp in the word just below the loader (base − 2, where the hand-over jumps). Every other vector stays the application's. The patched page 0 and the trampoline page are written first, so from the first write on an interrupted flash still resets into the loader; an erase runs top-down for the same reason.

Updating the loader

pureboot.py --update-loader new_pureboot.bin replaces the resident loader with any pureboot build — a re-timed window, a newer protocol — using the loader itself as its own staging loader. The image is the loader's own 512 bytes as a raw binary, or the Intel HEX the build emits beside it, which links the loader at its base inside an otherwise blank flash image:

1. The staging slot [base−512, base) is saved to a host-side state file (on the 1 KB tiny13A that is the whole application, vectors included).
2. The resident installs the identical update image there. On the tinies the host composes the slot's last word — the same address as the resident's trampoline — as a jump to the resident base, so even an abandoned staging copy times out into a loader, never into garbage.
3. J enters the staging copy, which rewrites the resident slot. On the t85 the host first re-aims word 0 at the staging copy, so a power loss mid-rewrite still resets into a loader; on the t13a the staging slot carries the reset vector itself.
4. J enters the new resident, which restores the staging slot's saved content (word 0 and the trampoline with it) and the state file is discarded.

Every phase is idempotent and keyed off the actual flash state: re-running the same command after any interruption resumes and completes. The state file carries the only bytes not recoverable from the device; if it is lost mid-update the update still completes, and the staging region is restored by reflashing the application. The mega needs its fuses for the preflight (BOOTSZ gate, profile notes) — read from the device, or supplied with --assume-fuses where reading is impossible (simulators).

Host tool

pureboot.py — Python 3, standard library only. The port layer is the one platform-specific part: termios drives any tty on POSIX (a USB adapter as well as a simavr pty), the Win32 serial API through ctypes drives a COM port on Windows (--port COM6; the \\.\ form for two-digit ports is supplied by the tool). Opening the port asserts DTR and RTS on both, so a board that wires DTR to reset gets its reset pulse and opens the activation window by itself.

pureboot.py --port /dev/ttyUSB0 --baud 57600 \
    --info --fuses --flash app.hex

Operations run in a fixed order within one session: info, fuses, loader update, flash (erase / program / read / verify), EEPROM (erase / program / read / verify) — then the loader hands over to the application; --stay keeps the session alive instead, and a later invocation reconnects into it (the knock converges there too). --flash and --eeprom verify by read-back unless --no-verify; images are raw binary, or Intel HEX by extension. --force overrides the refusable safety checks (today: flashing application data into a mega's reset walk region).

Tests

Per chip preset, ctest runs:

• pureboot.size — the 510-byte (tinies) / 512-byte (mega) budget;
• pureboot.pi — the position-independence lint: no absolute jmp/call in the image, the info block within its first 256 bytes;
• pureboot.planner — the host tool's pure logic: programming orders and their recovery properties, the surgery, the staging composition, the boot-fuse decode, and the update preflight's error/warning matrix over synthetic fuse bytes;
• pureboot.protocol — end to end against a simavr device (test/pureboot_device.c — the mega's USART as a pty; on the tinies a cycle-timed GPIO⇄pty bridge for the software UART, plus the SPM/NVM module simavr's tiny cores lack) driven by the real host tool through knock-from-reset, program + verify of both memories, session reconnect, an external reset through the patched vector, and the hand-over to a fixture application whose banner proves the launch — cross-checked against the simulator's ground-truth memory dumps and an independent decode of the surgery's rjmp words;
• pureboot.reloc — the identical image installed one slot below the resident serves the complete command set from there (the position-independence acceptance test);
• pureboot.update — the full --update-loader flow to a re-timed build, 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 throughout.

size, pi, and planner are host logic and run anywhere; the three simulator-driven targets need simavr and a pty, so they are POSIX-only — on Windows the tool is exercised against real hardware.