MegaCore

An Arduino core for most 64 and 100-pin AVRs, all running the Urboot bootloader. This core requires at least Arduino IDE v1.8, where v1.8.9 or newer is recommended. IDE 2.x should also work.

From MegaCore version 3 and onwards, the Optiboot bootloader has been replaced by the superior Urboot bootloader. It's smaller, faster, and has automatic baud rate detection, and can read and write to EEPROM. Other cool features the bootloader provides but are not utilized by MegaCore are user program metadata stored in flash (that can easily be viewed by Avrdude -xshowall) and chip-erase functionality. If you already have Optiboot installed and don't want to replace it with Urboot, you can still upload programs without any compatibility issues. However, if you're burning a bootloader to a new chip, Urboot is the way to go.

Table of contents

• Supported microcontrollers
• Supported clock frequencies
• Bootloader option
• Baud rate option
• BOD option
• EEPROM retain option
• Link time optimization / LTO
• Printf support
• Pin macros
• Write to own flash
• PROGMEM with flash sizes greater than 64kB
• Programmers
• How to install
  • Boards Manager Installation
  • Manual Installation
  • Arduino CLI Installation
  • PlatformIO
• Getting started with MegaCore
• Wiring reference
• Pinout
• Minimal setup

Supported microcontrollers:

• ATmega6490
• ATmega6450
• ATmega3290
• ATmega3250
• ATmega2561
• ATmega2560
• ATmega1281
• ATmega1280
• ATmega649
• ATmega645
• ATmega640
• ATmega329
• ATmega325
• ATmega169
• ATmega165
• ATmega128
• ATmega64
• AT90CAN128
• AT90CAN64
• AT90CAN32

(All variants - A, L, P, PA, PV, V)

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

	Mega2560	Mega1280	Mega640	Mega2561	Mega1281	Mega128 CAN128	Mega6490 Mega6450	Mega64 CAN64 Mega649 Mega645	Mega6490 Mega6450	CAN32 Mega329 Mega325	Mega169 Mega165
Flash	256kB	128kB	64kB	256kB	128kB	128kB	64kB	64kB	32kB	32kB	16kB
RAM	8kB	8kB	8kB	8kB	8kB	4kB	4kB	4kB	2kB	2kB	1 kB
EEPROM	4kB	4kB	4kB	4kB	4kB	4kB	2kB	2kB	1kB	1kB	512B
IO pins	70/86 *	70/86 *	70/86 *	54	54	53	68	53	68	53	53
Serial ports	4	4	4	2	2	2	1	2 / 1 **	1	2 / 1 **	1
PWM pins	15	15	15	8	8	7	4	7 / 4 **	4	7 / 4 **	4
LED pin	PB7	PB7	PB7	PB5	PB5	PB5	PB7	PB5	PB7	PB5	PB5

* Pin 70-85 is not broken out on the Arduino Mega. Make sure to check out the AVR style pinout for a more straightforward pinout.

** ATmega165/169/325/329/645/649 has one hardware serial port and four PWM outputs.

Supported clock frequencies

MegaCore supports a variety of different clock frequencies. Select the microcontroller in the boards menu, then select the clock frequency. You will have to hit "Burn bootloader" in order to set the correct fuses and upload the correct bootloader. This also has to be done if you want to change any of the fuse settings (BOD and EEPROM settings) regardless if a bootloader is installed or not.

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. The Urboot bootloader has automatic baud rate detection (except when using UART2 and UART3 on ATmega640/1280/2560), so UART uploads should work fine even though the oscillator is a little too fast or too slow.

Frequency	Oscillator type	Default upload speed (bootloader has auto-baud)	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
11.0592 MHz	External crystal/oscillator	115200	Great clock for UART communication with no error
9.216 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
6 MHz	External crystal/oscillator	57600
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

MegaCore lets you select which serial port you want to use for uploading. UART0 is the default port for all targets, but any hardware serial port may be used. 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 384 bytes of flash memory on ATmega165/169/325/3250/329/3290 and 512 bytes on ATmega64/128/645/6450/649/6490/1281/2560 and AT90CAN32/64/128.

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

Baud rate option

Since Urboot has automatic baud rate detection, the upload baud rate can be changed without having to re-flash the bootloader. The default baud rate setting will pick a suited baud rate that also works with the legacy Optiboot bootloader used in earlier MegaCore versions. The other baud rate options may or may not work, depending on the clock frequency and accuracy of the clock source. A rule of thumb is that "non-round" baud rates like 230400 works best with "non-round" clock speeds like 18.4320 MHz, while "round" ones like 16 MHz work best with "round" baud rates like 250000.

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:

ATmega640/1280/2560	ATmega1281/2561	ATmega3290/6490 ATmega3250/6450	ATmega165/169 ATmega325/329 ATmega645/649	ATmega64/128	AT90CAN32/64/128
4.3V	4.3V	4.3V	4.3V	4.0V	4.1V
2.7V	2.7V	2.7V	2.7V	2.7V	4.0V
1.8V	1.8V	1.8V	1.8V		3.9V
					3.8V
					2.7V
					2.6V
					2.5V
Disabled	Disabled	Disabled	Disabled	Disabled	Disabled

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.

Note that if you're using an ISP programmer or have the Urboot bootloader installed, data specified in the user program using the EEMEM attribute will be uploaded to EEPROM when you upload your program in Arduino IDE. This feature is not available when using the older Optiboot bootloader.

#include <avr/eeprom.h>

volatile const char ee_data EEMEM = {"Data that's loaded straight into EEPROM\n"};

void setup() {
}

void loop() {
}

Link time optimization / LTO

Link time optimization (LTO for short) optimizes the code at link time, usually making the code significantly smaller without affecting performance. You don't need to hit "Burn Bootloader" in order to enable or disable LTO. Simply choose your preferred option 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, MegaCore 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. This is disabled by default to save space but can be enabled using a build flag if using PlatformIO.

If you're using a serial port, simply use Serial.printf("Milliseconds since start: %ld\n", millis());. You can also use the F() macro if you need to store the string in flash. Other libraries that inherit the Print class (and thus support 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 map "Arduino pins" to the port and port number:

// Use PIN_PE0 macro to refer to pin PE0 (Arduino pin 0)
digitalWrite(PIN_PE0, HIGH);

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

Write to own flash

MegaCore uses the excellent Urboot bootloader, written by Stefan Rueger. Urboot supports flash writing within the running application, meaning that content from e.g. a sensor can be stored in the flash memory directly without needing 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 your 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 afterward.

PROGMEM with flash sizes greater than 64kB

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 64kB. If you want to store data in other sections, you can use PROGMEM1 (64 - 128kB), PROGMEM2 (128 - 192kB), or PROGMEM3 (192 - 256kB), depending on the chip you're using. Accessing this data is not as straightforward as with PROGMEM, but it's still doable:

const char far_away[] PROGMEM1 = "Hello from far away!\n"; // (64  - 128kB)
const char far_far_away[] PROGMEM2 = "Hello from far, far away!\n"; // (128 - 192kB)
const char far_far_far_away[] PROGMEM3 = "Hello from far, far, far away!\n"; // (192 - 256kB)

void print_progmem()
{
  uint8_t i;
  char c;

  // Print out far_away
  for(i = 0; i < sizeof(far_away); i++)
  {
    c = pgm_read_byte_far(pgm_get_far_address(far_away) + i);
    Serial.write(c);
  }

  // Print out far_far_away
  for(i = 0; i < sizeof(far_far_away); i++)
  {
    c = pgm_read_byte_far(pgm_get_far_address(far_far_away) + i);
    Serial.write(c);
  }
  // Print out far_far_far_away
  for(i = 0; i < sizeof(far_far_far_away); i++)
  {
    c = pgm_read_byte_far(pgm_get_far_address(far_far_far_away) + i);
    Serial.write(c);
  }
}

Programmers

Select your 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 oscillator is recommended.

How to install

Boards Manager Installation

This installation method requires Arduino IDE version 1.8.0 or greater.

• Open the Arduino IDE.
• Open the File > Preferences menu item.
• Enter the following URL in Additional Boards Manager URLs: https://mcudude.github.io/MegaCore/package_MCUdude_MegaCore_index.json
• Open the Tools > Board > Boards Manager... menu item.
• Wait for the platform indexes to finish downloading.
• Scroll down until you see the MegaCore entry and click on it.
• Click Install.
• After installation is complete close the Boards Manager window.

Manual Installation

Click on the "Download ZIP" button in the upper right corner. 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 "MegaCore" will show up.

Arduino CLI Installation

Run the following command in a terminal:

arduino-cli core install MegaCore:avr --additional-urls https://mcudude.github.io/MegaCore/package_MCUdude_MegaCore_index.json

PlatformIO

PlatformIO is an open-source ecosystem for IoT and embedded systems, and supports MegaCore.

See PlatformIO.md for more information.

Getting started with MegaCore

• 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, select MegaCore and select your preferred target.
• 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 preferred clock frequency. 16 MHz is standard on most Arduino boards.
• Select what kind of programmer you're using under the Programmers menu.
• Hit Burn Bootloader. The LED pin will not toggle after the bootloader has been loaded.
• Disconnect the ISP programmer, and connect a USB to serial adapter to the target microcontroller shown in the pinout diagram. Select the correct serial port under the Tools menu, and click the Upload button. If you're getting a timeout error, it may be because the RX and TX pins are swapped, or the auto-reset circuit isn't working properly (the 100 nF capacitor and a 10k resistor on the reset line).

Your code should now be running on the microcontroller!

Wiring reference

To extend this core's functionality a bit further, I've added a few missing Wiring functions to this hardware package. 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 further information please view the Wiring reference page

Pinout

64-pin chips

MegaCore provides a standard, logical pinout for the 64-pin chips. The standard LED pin is assigned to digital pin 13/PIN_PB5.
Click to enlarge:

ATmega64/128/1281/2561 AT90CAN32/CAN64/CAN128	ATmega165/169/325/329/645/649

100-pin chips

MegaCore provides a standard, logical pinout for the 64-pin chips. The standard LED pin is assigned to digital pin 13/PIN_PB5. MegaCore includes the original Arduino Mega pinout for the ATmega640/1280/2560, but also a straightforward, "standard" pinout for all 100-pin chips. The standard LED pin is assigned to digital pin 13 on the Arduino MEGA pinout and digital pin 22 on the standard pinout (PIN_PB7 in both cases).
Click to enlarge:

ATmega640/1280/2560 Arduino MEGA pinout	ATmega640/1280/2560 "AVR" pinout	ATmega3250/3290/6450/6490

Minimal setup

Here are some simple schematics showing a minimal setup using an external crystal. Omit the crystal and the two 22pF capacitors if you're using the internal oscillator.
Click to enlarge:

ATmega64/128/1281/2561 AT90CAN32/CAN64/CAN128	ATmega165/325/645 ATmega169/329/649	ATmega640/1280/2560	ATmega3250/6450 ATmega3290/6490