How Arduino bootloader works


Everyone that uses Arduino can tell you how easy it is to get projects going once they have an Arduino board in hand. Just load up the blink code and press upload. In a matter of seconds, your Arduino LED is blinking ever so confidently. But do you stop and wonder how Arduino receives the blink code? Enter Arduino bootloader. Every Arduino has a bootloader, a small program that is always stored on the Arduino to update the code in the Arduino. It only runs once per reset. It looks for new code to be loaded to the Arduino before starting the existing code.

The bootloader works like this:

On the PC side, to trigger a reset on the RESET pin, the PC (avrdude.exe or GNU/Linux equivalent) opens the serial port to Arduino when your press upload and the code is ready to upload. This causes the Data Transmit Ready (DTR) line of the USB/TTL chip to go LOW. The Arduino board has a capacitor charging circuit that uses this LOW (charging the capacitor) signal to momentarily pull down the RESET line of the ATMEGA328P chip before returning it to HIGH (capacitor charging completes). So Arduino resets each time its serial port is opened.

Upon reset, Arduino enters the bootloader.

The bootloader looks at the source that caused the reset. There are several sources that can cause a reset. If the reset was caused by the RESET pin, then it waits for one second for the PC to send in commands. When it receives valid commands, it will start accepting new Arduino code in HEX format and erase the existing code to load new one. If it doesn’t receive valid commands, it times out after one second and triggers a Watch Dog Timer (WDT) reset.

Once the bootloader runs again it will look at the source of the reset. Once it determines that it was the WDT reset, it immediately jumps to the first line of the actual code. This way if you power up Arduino, it will be able to immediately run your code, instead of waiting in the bootloader to time out 1 second. It’s pretty smart!

The optiboot bootloader is the most recent bootloader used by Arduino UNO, nano, and other boards based on ATMEGA328P and 1284P. It accepts commands or uploads code at 115,200, unlike the previous bootloader that accepts commands or uploads code at 57,600. A number of people have modified this standard bootloader to fit their own needs, such as slower upload speed for a bare-bone system with no crystals and runs at say 100KHz on internal RC oscillator to preserve battery. Others have made Ethernet upload possible. I’ve personally made a bootloader that will refuse to upload code unless a certain EEPROM byte is set to a certain value. This way if the device is a data logger and to be used by a student, the teacher won’t be worried that the student would get “smart” and erase the code. But when the teacher wants to change the code he/she can enter a password in the existing data logger program and unlock the device for upload again.

If you want to deploy a project, such as an artistic installation or a data logger, you can make a bare-bone system without the whole Arduino UNO board. Then you will need an ATMEGA328P chip with preloaded bootloader like this one, and some extra components.

Anyway, if your code doesn’t need to be changed any further, you can decide to get rid of the bootloader altogether. On an Arduino UNO, this will free up 512 bytes of FLASH and the end user of your device won’t have the danger of erasing the code. To load code without a bootloader, you can use Nick Gammon’s HEX uploader. All you need is another Arduino UNO and an SD card (shield format is preferred) and some jumper wires.

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