The Bootloader The behaviour described above happens thanks to a special piece of code that is executed at every reset of the microcontroller and that looks for a sketch to be uploaded from the serial/USB port using a specific protocol and speed. If no connection is detected, the execution is passed to the code of your sketch. This little (usually 512 bytes) piece of code is called the "Bootloader" and it is in an area of the memory of the microcontroller - at the end of the address space - that can't be reprogrammed as a regular sketch and had been designed for such purpose.
MemoryMap
The Memory Map of an ATmega328P
To program the bootloader and provide to the microcontroller the
compatibility with the Arduino Software (IDE) you need to use an In-circuit
Serial Programmer (ISP) that is the device that connects to a specific set
of pins of the microcontroller to perform the programming of the whole flash
memory of the microcontroller, bootloader included. The ISP programming
procedure also includes the writing of fuses: a special set of bits that
define how the microcontroller works under specific circumstances.
Use Arduino as ISP The whole process of loading the bootloader code and burning properly the fuses to make an ATmega microcontroller an "Arduino" is managed by the Arduino Software (IDE): it provides a specific menu item and allows you to use a variety of programming devices. Among the programmers, the "Arduino as ISP" is the cheapest and most practical solution to burn a bootloader on another Arduino board with ATmega, 32U4 or ATtiny.
ISP Programmers
The programming process uses VCC, GND and four data pins. Three pins connect
MISO, MOSI and SCK between the programming micro and the target micro, the
fourth pin from the programming micro goes to the reset pin of the target.
How to wire your boards The following table display on which pins the MOSI, MISO and SCK are broken out on the different Arduino boards:
ICSPHeader
On the Arduino UNO in the following image, we have highlighted in red the
connections on the female strips; in yellow the ICSP connector that connects
to the ATmega328P. Please note that the Rev.3 board has an ATMega 16U2 chip
that manages the USB connection and also that chip can be reprogrammed via a
dedicated connector labeled ICSP2, just above the ATMega 16U2 itself.
Uno Connect
On some Arduino boards (see table above), pins MOSI, MISO and SCK are the
same pins as digital pin 11, 12 and 13, respectively. That is why many
tutorials instruct you to hook up the target to these pins. If you find this
wiring more practical, have a define USE_OLD_STYLE_WIRING. This will work
even when not using an UNO. (With the UNO board this is not needed).
ArduinoUNOtoUNO ISP2
In the picture above we are connecting two UNO boards for bootloader burning
with the "old style" connections: the top board is the Target, the bottom
board is the Programmer. Note the yellow connection from D10 of the
programmer to RESET of the target. On MKR family of boards, you can't use
D10 for reset; we suggest D6 and you must remember to change the line 73 of
the ArduinoISP sketch -
#define RESET 10
- putting "6" instead of 10.
Arduino ISP wires
This Arduino NANO is programmed through its ICSP connector with wires coming
from D10-D13 of the programmer UNO board.
MegaToUNO
The Arduino MEGA above is programming an Arduino UNO connecting D51-D11, D50
-D12, D52-D13, GND-GND, 5V-5V and D10 to RESET. This type of board needs a
10µF electrolytic capacitor connected to RESET and GND with the positive
(long leg) connected to RESET. The capacitor has to be placed after the
programmer board has been loaded with the ISP sketch.
The 10µF electrolytic capacitor connected to RESET and GND of the
programming board is needed only for the boards that have an interface
between the microcontroller and the computer's USB, like Mega, UNO, Mini,
Nano. Boards like Leonardo, Esplora and Micro, with the USB directly managed
by the microcontroller, don't need the capacitor.
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KR1000 ISP UNO 2
In the above picture you see the wiring between a MKR1000 and a UNO. As
described above, everything runs on 3.3V, taken from VCC and GND of the
MKR1000 and sent to 5V and GND of the UNO. The same connection could be made
to the ICSP of the UNO following the pinout explained in the page. We have
used the same colors for the wires as in the other pictures to help you
switch from the "old wiring" to the ICSP connector with ease. Please note
that the MKR family of boards share the same pinout, therefore you can use
any MKR board as ISP programmer. If you use a MKR board as ISP programmer,
remember to change the line 73 of the ArduinoISP defining the actual pin
used on the MKR board to Reset the target (6 in the picture above).
Note: Please do not connect to USB or power supply the boards while you set
up the connections and wires. We also suggest that you first program the Arduino
used as ISP programmer before you proceed with the wiring to the target board.
Load the sketch
The Arduino that you will use as programmer needs a specific sketch. You
find it under Examples > 11. ArduinoISP > ArduinoISP .
Arduino ISP LEDSOK
To upload the sketch to your Arduino board - the one that you will use as
the programmer - you need to select board type and port, then you can
proceed as usual.
Program the bootloader
If all the wires are set, you need to switch to the board type you want to
program with the bootloader. This step is mandatory to select the proper
bootloader code and the fuses configurations. The programming procedure
checks the signature of the microcontroller before any writing action is
taken, but many boards share the same microcontroller and each board has its
own bootloader. The port remains the one of the ArduinoISP.
Choose "Burn bootloader" under tools and wait for the confirmation message
in the lower part of the Arduino Software (IDE) interface. If you connected
the LEDs you may follow the process visually.
Burn