px-lib  0.9.3
Cross-platform embedded library and documentation for 8/16/32-bit microcontrollers
XMODEM-CRC Bootloader

1. Introduction

This is an example of how to implement a bootloader on the Microchip AVR. It is a simple bootloader that accepts a binary application file transferred over the UART port using the XMODEM-CRC protocol.

A few optimizations were applied to decrease the size of the bootloader:

  • "CFLAGS += -ffunction-sections -fdata-sections" and "LDFLAGS += --gc-sections" so that unused functions will removed.
  • "LDFLAGS += -Wl,--relax" to enable linker relaxations, e.g. use RCALL assembly instruction instead of CALL where possible.
  • "LDFLAGS += -nostartfiles -nodefaultlibs -nostdlib" to remove Standard C Libraries.
  • Customized gcrt1.S (copied from avr-libc-1.8.0/crt1/gcrt1.S) added to remove interrupt vector table and .init9 section code.
  • The main function is declared with "__attribute__ ((section (".init9")))" so that it follows directly after the C Run Time (CRT) initialization.
  • The main function is also declared with "__attribute__ ((naked))" so that entry and exit code are ommitted.
  • The reset_vector function is declared with "__attribute__ ((noreturn))" so that the compiler will not add redundant code after the call to this "function".

In addition the following flags were passed to the linker:

  • "LDFLAGS += -Ttext=$(FLASH_BOOT_START)" to place the code at the start of the bootloader address, not the reset vector (0x0000).
  • "LDFLAGS += -Wl,--defsym,reset=0x0000" to define the address where the bootloader will jump to after it has finished execution.

2. How to use the XMODEM-CRC bootloader

2.1 Build the application and create a BIN file from the ELF file

Here is the part of the Makefile build that creates a BIN file from an ELF file:

avr-objcopy -O binary -R .eeprom -R .fuse -R .lock gpio.elf gpio.bin

2.2 Run and configure Tera Term - Terminal Emulator

Select the correct COM port and configure port for 115200 BAUD, 8 data bits, no parity, 1 stop bit and no flow control.

Setup > Serial port...

Tera Term COM port setup

At this stage you can verify that the communication between Tera Term and the board is OK, by resetting the board and verifying that Tera Term displays at least one received 'C' character sent by the bootloader.

2.3 Send the BIN file using the XMODEM-CRC protocol

File > Transfer > XMODEM > Send...

Tera Term - Start XMODEM file send

The BIN file (not HEX or ELF!) is selected and the CRC variant of XMODEM is specified.

Tera Term - Specify file and select CRC transfer method


Tera Term will now wait for a 'C' character from the board to start the transfer...

2.4 Reset the board to start the transfer

Press the reset button (RST) to reset the microcontroller and jump to the bootloader.

The bootloader will initiate the transfer and download the new application. At the end of the transfer, the bootloader will execute the new application.

Note that no safeguards are built into the bootloader. If the transfer is interrupted, the bootloader will time out and execute the broken, incomplete application. Normally this is not a big issue as the process can be repeated.

3. ATmega328p Fuse Bits

The AVR fuse bits must be set so that execution starts from the boot vector address. This means that the bootloader will always be executed first. The bootloader will send a 'C' character to start a transfer and waits for 1 second for a valid XMODEM-CRC data packet. If the transfer is not successful, it will jump to address 0x0000 and execute the application.

AVR fuse bits in Atmel Studio