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Build Status MegaCore forum thread

An Arduino core for ATmega8535, ATmega16, ATmega32, ATmega164, ATmega324, ATmega644 and ATmega1284, all running a Optiboot. Most Arduino UNO compatible libraries will work with this core. If not, it's fairly straight forward to port a library. This core requires at least Arduino IDE v1.6, where v1.8.7 or newer is recommended. If you're into "generic" AVR programming, I'm happy to tell you that all relevant keywords are being highlighted by the IDE through a separate keywords file. Make sure to test the example files (File > Examples > AVR C code examples).

If you're looking for a great development board for these DIP-40 microcontrollers, I got you covered! I've used the Arduino UNO for years, but felt like vital functionality was missing on the board. When designing this board I made sure all missing functionality was added. The board can be bought on my Tindie store.
Read more in the hardware section below.

Table of contents

Supported microcontrollers

  • ATmega1284
  • ATmega644
  • ATmega324
  • ATmega164
  • ATmega32
  • ATmega16
  • ATmega8535

* All variants - P, PA, A, PB. Select the correct version in the 'Variant' menu

Can't decide what microcontroller to choose? Have a look at the specification table below:

mega1284 mega644 mega324 mega164 mega32 mega16 mega8535
Flash 128kiB 64kiB 32kiB 16kiB 32kiB 16kiB 8kiB
RAM 16kiB 4kiB 2kiB 1kiB 2kiB 1kiB 0.5kiB
EEPROM 4kiB 2kiB 1kiB 0.5kiB 0.5kiB 0.5kiB 0.5kiB
Serial ports 2 2 2/3* 2 1 1 1
PWM pins 8 6 6/9* 6 4 4 4
IO pins 32 32 32/39* 32 32 32 32

* ATmega324PB has 3 serial ports, 9 PWM pins and 39 IO pins if internal oscillator is used.

EEPROM option

If you want the EEPROM to be erased every time you burn the bootloader or upload using a programmer, you can turn off this option. You'll have to connect an ISP programmer and hit "Burn bootloader" to enable or disable EEPROM retain. Note that when uploading using a bootloader, the EEPROM will always be retained.

Supported clock frequencies

MightyCore supports a variety of different clock frequencies. Select the microcontroller in the boards menu, then select the clock frequency. You'll have to hit "Burn bootloader" in order to set the correct fuses and upload the correct bootloader. Make sure you connect an ISP programmer, and select the correct one in the "Programmers" menu. For time critical operations an external crystal/oscillator is recommended.

You might experience upload issues when using the internal oscillator. It's factory calibrated but may be a little "off" depending on the calibration, ambient temperature and operating voltage. If uploading failes while using the 8 MHz internal oscillator you have these options:

  • Edit the baudrate line in the boards.txt file, and choose either 115200, 57600, 38400 or 19200 baud.
  • Upload the code using a programmer (USBasp, USBtinyISP etc.) or skip the bootloader by holding down the shift key while clicking the "Upload" button
  • Use the 4, 2 or 1 MHz option instead
Frequency Oscillator type Speed Comment
16 MHz External crystal/oscillator 115200 Default clock on most AVR based Arduino boards
20 MHz External crystal/oscillator 115200
18.4320 MHz External crystal/oscillator 115200 Great clock for UART communication with no error
14.7456 MHz  External crystal/oscillator 115200 Great clock for UART communication with no error
12 MHz External crystal/oscillator 57600 Useful when working with USB 1.1 (12 Mbit/s)
11.0592 MHz External crystal/oscillator 115200 Great clock for UART communication with no error
8 MHz External crystal/oscillator 57600 Common clock when working with 3.3V
7.3728 MHz External crystal/oscillator 115200 Great clock for UART communication with no error
4 MHz External crystal/oscillator 9600
3.6864 MHz External crystal/oscillator 115200 Great clock for UART communication with no error
2 MHz External crystal/oscillator 9600
1.8432 MHz External crystal/oscillator 115200 Great clock for UART communication with no error
1 MHz External crystal/oscillator 9600
8 MHz Internal oscillator 38400 Might cause UART upload issues. See comment above
4 MHz Internal oscillator 9600 Derived from the 8 MHz internal oscillator
2 MHz Internal oscillator 9600 Derived from the 8 MHz internal oscillator
1 MHz Internal oscillator 9600 Derived from the 8 MHz internal oscillator

Bootloader option

MightyCore lets you select which serial port you want to use for uploading. UART0 is the default port for all targets, but ATmega324/644/1284 can also use UART1 for upload. If your application doesn't need or require a bootloader for uploading you can also choose to disable it by selecting No bootloader. This frees 512 bytes of flash memory on ATmega8535/16/32/164/324 and 1024 bytes on ATmega644/1284.

Note that you have need to connect a programmer and hit Burn bootloader if you want to change any of the Bootloader settings.

BOD option

Brown out detection, or BOD for short lets the microcontroller sense the input voltage and shut down if the voltage goes below the brown out setting. To change the BOD settings you'll have to connect an ISP programmer and hit "Burn bootloader". Below is a table that shows the available BOD options:

ATmega1284 Atmega644 ATmega324 ATmega164 ATmega32 ATmega16 ATmega8535
4.3V 4.3V 4.3V 4.3V 4.0V 4.0V 4.0V
2.7V 2.7V 2.7V 2.7V 2.7V 2.7V 2.7V
1.8V 1.8V 1.8V 1.8V - - -
Disabled Disabled Disabled Disabled Disabled Disabled Disabled

Link time optimization / LTO

Link time optimization (LTO for short) has been supported by the IDE since v1.6.11. The LTO optimizes the code at link time, making the code (often) significantly smaller without making it "slower". In Arduino IDE 1.6.11 and newer LTO is enabled by default. I've chosen to disable this by default to make sure the core keep backward compatibility. Enabling LTO in IDE 1.6.10 and older will return an error. I encourage you to try the new LTO option and see how much smaller your code gets! Note that you don't need to hit "Burn Bootloader" in order to enable LTO. Simply enable it in the "Tools" menu, and your code is ready for compilation. If you want to read more about LTO and GCC flags in general, head over to the GNU GCC website!

Printf support

Unlike the official Arduino cores, MightyCore has printf support out of the box. If you're not familiar with printf you should probably read this first. It's added to the Print class and will work with all libraries that inherit Print. Printf is a standard C function that lets you format text much easier than using Arduino's built-in print and println. Note that this implementation of printf will NOT print floats or doubles. If you need printf to print floats, you'll have to use PlatformIO.

If you're using a serial port, simply use Serial.printf(F("Milliseconds since start: %ld\n"), millis());. As you can see, printf supports the F() macro, but you don't have to use it. Other libraries that inherit the Print class (and thus supports printf) are the LiquidCrystal LCD library and the U8G2 graphical LCD library.

Pin macros

Note that you don't have to use the digital pin numbers to refer to the pins. You can also use some predefined macros that maps "Arduino pins" to the port and port number:

// Use PIN_PB0 macro to refer to pin PB0 (Arduino pin 0 with the standard and sanguino pinout)
digitalWrite(PIN_PB0, HIGH);

// Results in the exact same compiled code
digitalWrite(0, HIGH);

Write to own flash

MightyCore uses Optiboot Flash, a bootloader that supports flash writing within the running application, thanks to the work of @majekw. This means that content from e.g. a sensor can be stored in the flash memory directly without the need of external memory. Flash memory is much faster than EEPROM, and can handle at least 10 000 write cycles before wear becomes an issue. For more information on how it works and how you can use this in you own application, check out the Serial_read_write for a simple proof-of-concept demo, and Flash_put_get + Flash_iterate for useful examples on how you can store strings, structs and variables to flash and retrieve then afterwards. The Read_write_without_buffer example demonstrate how you can read and write to the flash memory on a lower level without using a RAM buffer.

PROGMEM with flash sizes greater than 64kiB

The usual PROGMEM attribute stores constant data such as string arrays to flash and is great if you want to preserve the precious RAM. However, PROGMEM will only store content in the lower section, from 0 and up to 64kiB. If you want to store data in the upper section, you can use PROGMEM1 (64 - 128kiB) if your target is an ATmega1284/P. Accessing this data is not as straight forward as with PROGMEM, but it's still doable:

const char far_away[] PROGMEM1 = "Hello from far away!\n"; // (64  - 128kiB)

void print_progmem()
  char c;

  // Print out far_away
  for(uint8_t i = 0; i < sizeof(far_away); i++)
    c = pgm_read_byte_far(pgm_get_far_address(far_away) + i);


This core has three different pinout options:

  • Standard: The default pinout, and is based on the original AVR pinout.
  • Bobuino: Basically an Arduino UNO pinout setting. This pinout version is great for using with shields or code that's written for the Arduino UNO, as the pin functions stay the same (MOSI on D11, MISO on D12, SCK on D13).
  • Sanguino: This pinout is common on older 3D printer controllers such as the Sanguino, RepRap Sanguinololu, and RepRap Gen7. This pinout is also known as "avr_developers".

Please have a look at the (pins_arduino.h) files for detailed info. Pick your favorite!

Click to enlarge:

MightyCore Standard pinout MightyCore Bobuino pinout MightyCore Sanguino pinout


MightyCore adds its own copies of all the standard programmers to the "Programmer" menu. You must select the MightyCore copy of the programmer you are using for "Upload Using Programmer" to work with ATmega1284, ATmega324A, ATmega324PB, or ATmega164A.

How to install

Boards Manager Installation

This installation method requires Arduino IDE version 1.6.4 or greater.

  • Open the Arduino IDE.
  • Open the File > Preferences menu item.
  • Enter the following URL in Additional Boards Manager URLs:
  • Separate the URLs using a comma ( , ) if you have more than one URL
  • Open the Tools > Board > Boards Manager... menu item.
  • Wait for the platform indexes to finish downloading.
  • Scroll down until you see the MightyCore entry and click on it.
  • Click Install.
  • After installation is complete close the Boards Manager window.

Manual Installation

Click on the "Download ZIP" button. Extract the ZIP file, and move the extracted folder to the location "~/Documents/Arduino/hardware". Create the "hardware" folder if it doesn't exist. Open Arduino IDE, and a new category in the boards menu called "MightyCore" will show up.


If you plan to use the ATmega324PB you need the latest version of the Arduino toolchain. This toolchain is available through IDE 1.8.6 or newer. Here's how you install/enable the toolchain:

  • Open the Tools > Board > Boards Manager... menu item.
  • Wait for the platform indexes to finish downloading.
  • The top is named Arduino AVR boards. Click on this item.
  • Make sure the latest version is installed and selected
  • Close the Boards Manager window.


PlatformIO is an open source ecosystem for IoT development and supports MightyCore.

See for more information.

Getting started with MightyCore

Ok, so you've downloaded and installed MightyCore, but how do you get the wheels spinning? Here's a quick start guide:

  • Hook up your microcontroller as shown in the pinout diagram.
    • If you're not planning to use the bootloader (uploading code using a USB to serial adapter), the FTDI header and the 100 nF capacitor on the reset pin can be omitted.
  • Open the Tools > Board menu item, and select a MighyCore compatible microcontroller.
  • If the BOD option is presented, you can select at what voltage the microcontroller will shut down at. Read more about BOD here.
  • Select your prefered pinout. Personally I prefer the standard pinout because it's "cleaner", but the Bobuino pinout is better at Arduino UNO pin compatibility. Read more about the different pinouts here.
  • Select your prefered clock frequency. 16 MHz is standard on most Arduino boards.
  • Select what kind of programmer you're using under the Programmers menu.
  • If the Variants option is presented, you'll have to specify what version of the microcontroller you're using. E.g the ATmega1284 and the ATmega1284P have different device signatures, so selecting the wrong one will result in an error.
  • Hit Burn Bootloader. If an LED is connected to pin PB0, it should flash twice every second.
  • Now that the correct fuse settings is set and the bootloader burnt, you can upload your code in two ways:
    • Disconnect your programmer tool, and connect a USB to serial adapter to the microcontroller, like shown in the minimal setup circuit. Then select the correct serial port under the Tools menu, and click the Upload button. If you're getting some kind of timeout error, it means your RX and TX pins are swapped, or your auto reset circuity isn't working properly (the 100 nF capacitor on the reset line).
    • Keep your programmer connected, and hold down the shift button while clicking Upload. This will erase the bootloader and upload your code using the programmer tool.

Your code should now be running on your microcontroller! If you experience any issues related to bootloader burning or serial uploading, please use this forum post or create an issue on Github.

Wiring reference

To extend this core's functionality a bit futher, I've added a few missing Wiring functions. As many of you know Arduino is based on Wiring, but that doesn't mean the Wiring development isn't active. These functions are used as "regular" Arduino functions, and there's no need to include an external library.
I hope you find this useful, because they really are!

Function list

  • portMode()
  • portRead()
  • portWrite()
  • sleepMode()
  • sleep()
  • noSleep()
  • enablePower()
  • disablePower()

For more information please view the Wiring reference page!

Library porting

Some users have reported issues when trying to use some 3rd party libraries with the ATmega8535, ATmega16 or ATmega32. A simple guide on how to port a library can be found here.


I've designed a development board for this particular core. I've added all the functionality I missed with the original Arduino boards, and added the original AVR pinout. Not all supported microcontrollers have the same pin functions, and differences are highlighted. The boards measures 8.0 * 10.0 cm (3.15 * 3.94 in)
The development board has some additional unique features:

  • A voltage select jumper to run the microcontroller at 5V or 3.3V
  • A breadboard friendly AVR with 32 IO pins, including 8 analog inputs
  • All pin located at the same side of the board, making it easy to hook it up to a breadboard
  • Male and female IO pin headers
  • Plenty of 5V, 3.3V and GND points broken out, both male and female
  • A large ground pad on the underside of the board for connecting alligator clips, such as the ground clip of your oscilloscope
  • A potentiometer for using as a voltage reference (e.g adjusting the LCD contrast)
  • Onboard LED connected to digital pin 0 (PB0)
  • A socketed crystal, perfect for experimenting with different clock frequencies
  • An auto reset enable header if you don't want the microcontroller to be reset every time you open the serial monitor on your PC
  • PWM pins clearly marked and a lookup table that can be found on the under side of the board (three ~ - all microcontrollers, two ~ - 164; 324; 644; 1284, one ~ - 1284)
  • IO peripherals written on the underside of the board. No need to search in the datasheet anymore!
  • A Mini USB connector instead of a large USB Type-B plug
  • All serial hand shake pins broken out for applications such as bit banging (CTS, DTR, RI, DCD, DRT, RST)
  • JTAG header for programming and debugging (the JTAG enable fuse must be sat first)

The development board can be bought on my Tindie store. This includes a pre programmed ATmega32 chip.

Click the images for full resolution

Development board front

Development board back

Bottom row

Minimal setup

Here is a simple schematic showing a minimal setup using an external crystal. Skip the crystal and the two capacitors if you're using the internal oscillator.
Click to enlarge:

DIP-40 package TQFP-44 SMD package ATmega324PB SMD package