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Arduino Uno

Support for the Arduino Uno board. More...

Detailed Description

Support for the Arduino Uno board.


The Arduino Uno is one of the cheapest board to start and discover with electronics and embedded coding. It is based on Atmel's AVR architecture and sports an ATmega328p MCU. It is like many Arduinos extensible by using shields.


MCU ATmega328p
Family AVR/ATmega
Vendor Atmel/Microchip
Flash 32 KiB
Frequency 16 MHz
Timers 3 (2x 8bit, 1x 16bit)
ADCs 6 analog input pins
SPIs 1
I2Cs 1 (called TWI)
Vcc 5.0 V
Datasheet / Reference Manual Datasheet and Reference Manual
Board Manual Board Manual

Flashing the device

Flashing RIOT on the Arduino Uno is quite straight forward, just connect your Arduino Uno using the programming port to your host computer and type:

make BOARD=arduino-uno flash

This should take care of everything!

We use the open avrdude tool to write the new code into the ATmega328p's flash

Pin Change Interrupts

More pins can be used for hardware interrupts using the Pin Change Interrupt feature. See ATmega common for details.


Don't expect having a working network stack due to very limited resources.

On-Chip Debugging

On-Chip Debugging on the Arduino Uno is not supported via the usual JTAG interface used in ATmega MCUs with higher pin counts, but via debugWIRE. While debugWIRE has the advantage of only using the RESET pin to transfer data, the features provided are extremely limited. If the same issue can be reproduced on an Arduino Mega2560, which supports JTAG, it will be much easier and more productive to debug your code on the Arduino Mega2560. If the bug cannot be reproduced, limited on chip debugging is possible on the Arduino Uno nonetheless.


Debugging Hardware

In order to be able to use On-Chip Debugging you will need the AVR Dragon, which is the cheapest least expensive programmer and debugger available that supports programming via SPI ("normal ISP"), High Voltage Serial Programming, and Parallel Programming, as well as debugging via JTAG, debugWIRE, PDI and aWire. So at least can use it for just about every AVR device.

Board Modifications

On the Arduino Uno the RESET pin of the MCU is connected to a 100 nF capacitor, which in turn is connected to the ATmega16U2 (or an CH340 TTL Adapter in case of most clones). This allows the device to be automatically reset when you connected to the board via a serial. This is particularly useful during programming via the bootloader (without external ISP programmer), as avrdude can trigger the reset and, thus, start the bootloader without the user having to press a button.

In order to use on-chip debugging, the capacitor needs however to be disconnected from the reset pin. With the original Arduino Uno this can be done by cutting the solder jumper labeled "RESET EN". This can easily be resoldered to restore the original state. Most clones do not have this solder jumper and you will likely have to break off the usually surface mounted capacitor. A multimeter can be used to detect which capacitor is connected to the reset pin. Keep in mind that the capacitor will likely be destroyed when removed by force and it will be difficult to restore the auto-reset feature of the clones.


You need to have AVaRICE installed. Some distros have this packaged already. If you need to compile it by hand, go for the latest SVN revision. The latest release cannot be compiled on anything but historic platforms and contains bugs that prevent it from debugging the ATmega328P anyway.


In order to use On-Chip Debugging, the DWEN bit in the high fuse needs to be enabled (set to zero). The exact fuse settings for debugging and the default fuse setting are these:

Fuse Default Setting Debug Setting
Low Fuse 0xFF 0xFF
High Fuse 0xDE 0x9E
Extended Fuse 0xFD 0xFD

You can enable debugWIRE debugging by running (replace <PROGRAMMER> by the name of your programmer, e.g. dragon_isp in case of the AVR Dragon):

avrdude -p m328p -c <PROGRAMMER> -U hfuse:w:0x9e:m

And disable debugging via:

avrdude -p m328p -c <PROGRAMMER> -U hfuse:w:0xde:m
You can use a different ISP to enable debugging, but disabling it again will only work with the AVR Dragon: The ISP will require the RESET pin to work, but the RESET pin is re-purposed for debugWIRE when debugging is enabled. Recent versions of avrdude will use the debugWIRE interface to temporarily disable debugWIRE and restore the RESET pin's default behavior in order to use the ISP. But this requires a programmer/debugger that can be used as both ISP and debugWIRE debugger using the same connector. So don't enable debugging unless you have an AVR Dragon or another plan on how to disable debugging again.


With the AVR Dragon, debugging is as simple as running:

make BOARD=arduino-uno debug
For flashing the device via ISP, avrdude will temporarily disable debugWIRE. If AVaRICE complains that synchronization with the device is not possible after having it flashed, the device might need a cold boot to enable debugWIRE again.

The memory map of the ELF file does not take the bootloader into account. The author of this text used an ISP to program the Arduino Uno during debugging to avoid any issues. You might want to do the same, e.g. via:

make BOARD=arduino-uno PROGRAMMER=dragon_isp flash
Flashing via ISP overwrites the bootloader. But you can restore it easily using the ISP. Consult the Arduino documentation on how to restore the bootloader.
If you are using a different debugger than the AVR Dragon, you have to export the AVR_DEBUGDEVICE environment variable to the required flag to pass to AVaRICE, e.g. when using the Atmel-ICE you have to export AVR_DEBUGDEVICE=--edbg. If the debug device is not connected via USB, you also need to export AVR_DEBUGINTERFACE to the correct value.

Breakpoints / Watchpoints

The ATmega328P only has a single hardware break point and zero watchpoints. The single hardware breakpoint is used for single-stepping. As a result neither breakpoints nor watchpoints can be used. AVaRICE tries to emulate breakpoints be inserting the break machine instruction into the ROM in place of the original instruction to break on. Once this break instruction is reached, the original instruction is restored. This is not only super slow, but also wastes two flash cycles every time a breakpoint is hit. This cumulates to significant flash wear during long debugging sessions.


file  board.h
 Board specific definitions for the Arduino Uno board.