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TFTP Bootloader

I'm working on a project of remotely operated astronomic observatory, and in the scope of this project I needed many TCP/IP controlled boards.

As I'm familiar with networks environements the availability of a working and reliable TFTP bootloader was really missing to the Arduino boards in my point of view.

So I decided to rewrite the code of the unachieved TFTP bootloader published by the Arduino team, and since a few weeks I have a bootloader that works correctly and helps me in my work on the Arduino boards in development environment and production.

As I think this feature is very nice (fast, reliable and network oriented) I think it is interesting to publish it, and maybe to add a feature in the IDE in order to program the Arduino directly trough TFTP.


Actually you can download it from this website:


Compilation and versions

Author: Denis PEROTTO, from work published by Arduino
License: GPL
Project: TFTP Bootloader
Compilator : gcc version 4.3.3 (WinAVR 20100110)
Version: 0.2 tftp functional on 328P
Ethernet bootloader for the ATmega328P / W5100

How it works

This bootloader starts a TFTP server on default UDP Port 69, and listens for about 5 seconds at startup. During this time the user can upload a new firmware using any TFTP client software.

It manages interrupted file transfers even if a power failure succeed.


At this time the bootloader's size is about 1500 words (3000 Bytes). Fuses have to be configured in order to declare a 2Kwords booloader.
Therefore the user code must not exceed 28672 Bytes.

Supported Hardware

At this time the bootloader has only been tested on Arduino Ethernet, but the source code can easily be adapted to other boards.
The code to be modified is:

  • The beginning of the net.h file for W5100 SPI interface configuration:

#define SCK_PIN 5 //PB5
#define MISO_PIN 4 //PB4
#define MOSI_PIN 3 //PB3
#define SS_PIN 2 //PB2

  • The beginning of main.h for the LED pin configuration:

#define LED_PIN 1

There is only one supported ethernet controller, the W5100.


Bytes 0 to 20 are reserved for the TFTP Bootloader, please read "Settings stored in flash" below

Flashing bootloader:

Connect a TinyUSB programmer and "make install".

Configuring your network:

  • The bootloader default address is
  • Configure your computer network card to a static address of 192.168.1.x with a subnet of
  • By default, the bootloader assumes an internet gateway is at address

Converting firmware to the right format:

The bootloader accepts raw binary images, starting at address 0x0000.
These can be generated using avr-objcopy, part of WinAVR / AVR-GCC, using the "-O binary" option.

Example: avr-objcopy -j .text -j .data -O binary [firmware].elf [firmware].bin

When using the Arduino IDE you will need to make a verbose compile of your sketch and then check in the temp directory use for the .elf file freshly generated.

Uploading firmware manually

  1. Check the target board is powered, and connected to the computer ethernet.
  2. Verify the computer network settings: Static IP of 192.168.1.x, Subnet of
  3. Push reset button to start the bootloader. The LED will blink rapidly.
  4. In a console window (Windows): tftp -i put [firmware].bin
  5. The board will be reprogrammed and reboot.

Flash codes:

Rapid blinking: Ethernet bootloader is wainting for a connection.


  • Timeout at startup is set to 5 seconds approximately
  • Timeout will never occur if there is no firmware in user flash space
  • Timeout (also 5 seconds) will occur if a download has been started and no response is sent by the client, and TFTP server will restart and wait for a new attempt
  • Timeout will never occur after an hardware reset if there has been an interrupted transfer before (we suppose the image in flash to be incomplete)

IP Addressing:

This TFTP bootloader does not implement a DHCP client at this time. As it has been thought to be as tiny, reliable and autonomous as possible only static addressing is supported.

DHCP boot with TFTP server address recovery in specific DHCP Option could be an option in order to have a diskless station like behaviour, but once again this bootloader has been thought to be rock-solid and automatic flashing of firmware at startup is a dangerous option.

Settings stored in flash:

In order to provide the possibility to configure the Ethernet settings used by the TFTP Bootloader and the user application, the following settings can be stored in the MCU flash.

These are intended to be shared between the TFTP Bootloader and the client App. This will avoid any inconsistencies and ARP resolution problems during the initialisation process that would succeed if different MAC addresses are used for example.

  • 0:SIG1(0x55), 1:SIG2(0XAA) --> Signature, if present Bootloader will use the following settings for configuring the Ethernet Chip

  • 2:IMAGE STATUS : 0xFF: BAD or never uploaded, 0xBB: IMAGE OK
  • 3:GWIP0, 4:GWIP1, 5:GWIP2, 6:GWIP3 --> Gateway IP address (Default
  • 7:MASK0, 8:MASK1, 9:MASK2, 10:MASK3 --> Mask (Default
  • 11:MAC0, 12:MAC1, 13:MAC2, 14:MAC3, 15:MAC4, 16:MAC5 --> MAC Address (Default 12:34:45:78:9A:BC)
  • 17:IP0, 18:IP1, 19:IP2, 20:IP3 --> IP Address (Default

The consistency of these settings have to be garanteed by the client application.

Errors handled during TFTP transfer:

The following error codes and error strings can be reported to the TFTP client software:

  • "Opcode?", Code 1, TFTP Opcode sent by client is not supported
  • "Flash Full", Code 3, Flash size exceeded
  • "Invalid Image", Code 5, Firmware sent by client don't start with 0x0C 0x94 0xWX 0xYZ (JMP 0xWXYZ)
  • "Error", Code 0, Unknown error


Debugging can be enabled at time of compilation in debug.h, uncomment the following line, save and compile:

//#define DEBUG

should be replaced by :

  1. define DEBUG

Debug will be outputted to the serial port of the Arduino board at 115200 bps speed.

Version history

  • 0.1 (xx/xx/xxxx): First internal release. Supports uploads on tftp. (Arduino unachieved published release)
  • 0.2 (07/12/2011): Reviewed by Denis PEROTTO, comments added, additionnal error codes added and code cleaned

Todo List

  • Return to Ethernet default settings when putting one pin HIGH during startup
  • Enable TFTP debugging by setting a value in flash in the client application
  • Add syslog functionnality in addition to serial debugging
  • Add possibility to configure the timeout of the TFTP bootloader by setting a value in flash in the client application


For any questions you can contact me at denis dot perotto at gmail dot com