Feature-rich small AVR bootloader using urprotocol

• Tight code: most bootloaders fit into
  • 256 bytes albeit normally without EEPROM read/write support
  • 384 bytes with EEPROM read/write support
  • 512 bytes with dual-boot, EEPROM read/write support and Chip Erase capability
• Highly modular feature set:
  • Dedicated protocol between programmer and bootloader (urprotocol)
  • EEPROM read/write support
  • Vector bootloader for devices without a boot section or to save space on devices with one
  • Software UART: no hardware UART required, but also useful when the chip's CPU frequency and the bootloader's desired baud rate are incompatible (eg, 8 MHz and 115,200 baud)
  • Subroutine pgm_write_page(sram, progmem) for applications so they can change themselves: on many MCUs the SPM write flash only works when called from the bootloader section (default on)
  • Dual programming from external SPI flash memory for over-the-air programming
  • Template bootloader with nops that can be replaced with opcodes to set/clear the right LED or CS pins
  • Saves the reset flags in R2 for inspection by the application via the .init0 section (default on)
  • Bootloader protects itself from overwriting (default on)
  • Automatic host baud rate detection
  • Chip erase in bootloader (faster than -c urclock emulation)
  • Update flash feature that tries to wear out flash less (and is faster)
• Avrdude (from v7.1 onwards) supports urboot bootloaders with -c urclock

Urboot bootloaders can be hundreds of bytes shorter than optiboot with the same functionality, see below.

Background. More information on how bootloaders work with tips, tricks and limitations here.

Compiling an urboot bootloader. The small sizes of urboot bootloaders depend on many a trick of the trade, most of which only work with the avr-gcc toolchain used for compilation. It turns out that two older avr-toolchains, version 4.8.1 and 5.4.0, produce pretty tight code. Linux binaries of these are located in the src/avr-toolchain directory. This project uses helper programs in perl: an avr-gcc wrapper urboot-gcc and hexls that shows stats of the produced .hex files. So, all things considered, compiling this project is easiest on Linux with perl installed (and some required perl libraries, too; install them with cpan). All going well make all should produce .hex, .elf and .lst files of a number of bootloaders. The created .hex files should coincide with the corresponding ones in the directory src/all. Once all these hurdles are taken, it is easy to create own bootloaders with commands such as

 $ make MCU=atmega328p AUTOBAUD=1 VBL=1 AUTOFRILLS=5..10 NAME=atmega328p_a

Alternatively, the Dockerfile may be built and used to build binaries on other systems:

 $ docker run --platform linux/amd64 -v "$(pwd)/src":/src --rm -it $(docker build -q .) \
   MCU=atmega328p AUTOBAUD=1 VBL=1 AUTOFRILLS=5..10 NAME=atmega328p_a

More detailed information here: make options

Pre-compiled bootloaders. If compiling bootloaders is not feasible, feel free to try out one of the 1,283,806 different pre-compiled bootloaders in the directory tree for MCU parts. You can find bootloaders for popular boards in a corresponding directory for boards, for example, autobaud bootloaders for the venerable ATmega328P based Uno with a LED on PB5, the Arduino Pro Mini (same bootloaders), the Jeenode with a low-active LED on PB1, the ATmega1284P based Moteino Mega and the ATmega2560 Mega R3 board; or, eg, some 16 MHz and 115,200 baud bootloaders for the ATtiny167 based Digispark Pro, the ATtiny85 based Disgispark and the ATtiny84 based Luminet. Finally, there are pre-compiled urboot bootloaders for popular Arduino cores in the cores directory of the urboot.hex repository.

How to select a bootloader. Tips, tricks and some background here.

Usage. As with all bootloaders, one needs to first program them onto the board/chip using a (physical) programmer and an uploader program, eg, through

avrdude -c usbasp -p m328p -U flash:w:bootloader.hex:i

If the part does not have hardware support for bootloaders (eg, ATtiny167), then this is all that is needed. However, if your part has hardware support for bootloaders (eg, ATmega328p), then particular attention is needed as to whether the bootloader

• Is a vector bootloader (j), in which case the fuses need to be programmed so that on reset the MCU jumps to the reset vector at address 0. Ensure the lock bits do not preclude writing to any of the boot sections (otherwise the space freed up by vector bootloaders cannot be used).
• Assumes hardware support (h) and sits in a dedicated HW boot section, in which case the fuses need to be set to ensure that on reset the MCU jumps to the correct bootloader start; the boot section size fuse bits need to match the actual bootloader size (consult the data sheet of the part); the lock bits should protect the boot section from being written to.

Strictly speaking vector bootloaders also need a jmp or rjmp from the reset vector at address 0 to the bootloader. However, if the chip was erased before putting the bootloader on, then this is not necessary: Erased words read 0xffff, and although not an official opcode, behave as sbrs r31,7 (skip one instruction if bit 7 in R31 is set). A reset to address 0 on an otherwise erased flash will therefore eventually run into the start of the bootloader. Uploading the first application with avrdude -c urclock will set the reset vector correctly to jump to the bootloader.

Some parts also need the SELFPRGEN fuse set allowing the vital spm opcode (store to program memory) to be executed without which bootloaders cannot write to flash.

Once the board has its bootloader the board can be directly connected to and programmed from the host/laptop/PC (without a physical programmer), eg, through

 $ avrdude -c urclock -p m328p -b 115200 -P /dev/ttyUSB0 -U flash:w:mysketch.hex:i
 $ avrdude -c urclock -p m328p -b 115200 -P /dev/ttyUSB0 -U eeprom:r:backup.eep:I
 $ avrdude -c urclock -p m328p -b 115200 -P /dev/ttyUSB0 -t

Voila!

Fun facts. avrdude -c urclock can keep bootloaders in terminal mode -t without the bootloader resetting itself through the watchdog timer. avrdude -c urclock keeps metadata of the uploaded program, which are displayed with avrdude -c urclock -xshowall. Urboot bootloaders tell avrdude exactly which part they were compiled for; hence, -c urclock is the only programmer for which the part does not need specifying via -p.

Trouble shooting. So, you have selected the right bootloader for your board and followed above but programming does not work as expected?

• Carefully double check all fuse settings for your part; for example, if the bootloader requires hardware support then the bootloader size as given by command line tools or hexls will be in bytes but the data sheets often specify BOOTSZ in words (note the factor of 2).
• Check the lock settings for your part; for example, if the bootloader is a vector bootloader then the lock bits should enable to write to everywhere, even the hardware bootloader section. Urboot bootloaders are sometimes smaller than the smallest hardware boot sections and they protect themselves from being overwritten; if the hardware lock for a large hardware boot section is enabled then this space cannot be used for applications.
• Check the logical connections. If using a separate USB/serial converter check whether the correct I/O pins are hooked up: The RX pin that the bootloader was compiled with (typically, the RX pin of the processor's USART) needs to be connected to the TX pin (not the RX pin) of the USB/serial converter chip and vice versa. If the board has a USB/serial converter, check to which processor pins it is connected and verify these are the RX/TX pins that the bootloader was compiled with.
• Check the electrical connections. Some USB/serial converters require a hardware pullup on the TX side of the board. Some boards run at a different voltage than 5V USB; they may need their own power supply (in which case connect GND, but do not connect the different Vcc); they may need voltage level shifters.
• Check the mechanical connections. Do the contacts fit tight?
• Reset the board manually. Some boards have a hardware mechanism that allows avrdude to reset the board by plucking the RTS or the DTR line for a short while. Avrdude does both before starting communication, but if the board lacks the hardware for that, it needs to be reset manually before programming: First press the reset button of the board and release it; then start avrdude by pressing enter of the previously typed command line within the WDTO time (typically one second).
• Ensure neither the actual F_CPU nor the used baud rate deviates by more than 1%
  • Does the bootloader match the actual F_CPU frequency? Check the crystal on the board and/or the board documentation. Has the CKDIV8 fuse been programmed? Then the MCU runs slower by a factor of 8.
  • Does the avrdude baud rate match the bootloader's? Try an eighth of the nominal bootloader baud rate in case CKDIV8 is programmed. Autobaud bootloaders only recognise 256 different baud rates: F_CPU/8n where n is 1..256; is the used baud rate close to an auto-detectable one?
  • Measure the F_CPU of the board, eg, by measuring the blink frequency of the blink.hex sketch (uploaded with a physical programmer)
• Swap out the USB/serial cable if there is no USB/serial chip on board

Comparison. The table below lists a sample of urboot bootloaders and their features alongside vanilla optiboot and its larger bigboot variant for EEPROM access. They are all for a 16 MHz MCU and 115,200 baud serial communication speed, except for the autobaud ones, which should work for all F_CPU and a range of associated baudrates.

• Size: Bootloader code size including small table at top end
• Usage: How many bytes of flash are needed, ie, HW boot section or a multiple of the page size
• Version: For example, u7.6 is an urboot version, o5.2 is an optiboot version
• Features:
  • w bootloader provides pgm_write_page(sram, flash) for the application at FLASHEND-4+1
  • e EEPROM read/write support
  • U checks whether flash pages need writing before doing so
  • s uses skeleton of STK500v1 protocol (only u7.7); -c urclock and -c arduino both OK
  • d dual boot (over-the-air programming from external SPI flash)
  • h hardware boot section: make sure fuses are set for reset to jump to boot section
  • j vector bootloader: applications need to be patched externally, eg, using avrdude -c urclock
  • p bootloader protects itself from being overwritten
  • P vector bootloader only: protects itself and reset vector from being overwritten
  • r preserves reset flags for the application in the register R2
  • a autobaud detection (f_cpu/8n using discrete divisors, n = 1, 2, ..., 256)
  • c bootloader provides chip erase functionality (only recommended for large MCUs)
  • - corresponding feature not present

The naming convention for the .hex files is given in the larger comparison table covering a range of MCUs.