Features

• Breadboard friendly
• A little more minimal circuit than other currently available devboards/designs
• Designed for battery operation running at 8MHz at 3.3V
• MCP1700 or MCP1702 voltage regulators with very low quiescent current
• Draws about 5uA current when ATmega and NRF24L01+ are sleeping and GPIO pins 2 to 25 set to INPUT_PULLUP (1608 draws slightly less than 4808)
• Mostly 0805 footprints for easy soldering
• Should be compatible with megaAVR-0 ATmega808, 1608, 3208 and 4808 (tested with 1608 and 4808)
• The MCU used in this design is the same as on the AVR-IOT WG and similar to the one on the Arduino Nano Every (basically it only has less pins than the 4809)
• NRF24L01+ SMD module (optional)
• Use FTDI programmer (e.g. a cheap FTDI Basic 3.3V from Sparkfun) for flashing sketches and serial monitor in Arduino IDE, like on Arduino Pro Mini

WARNING: Do not connect a 5V FTDI board to the circuit when a NRF24L01+ is present in the circuit. Otherwise you may fry your NRF24L01+. Only use 3.3V boards

A cheap P-Channel MOSFET with low RDS(on) is used for reverse polarity protection, because unlike diodes it does not have voltage drop. MOSFET: AO3401A by Alpha & Omega Semiconductor Inc.

See this video about how this reverse polarity protection works:

https://www.youtube.com/watch?v=IrB-FPcv1Dc

Another OSHW board with the same MCU (but without FTDI connector) can be found here:

https://oshwlab.com/wagiminator/atmega4808-development-board_copy

Flashing the MegaCoreX bootloader through UPDI

Download the bootloader hex file from here:

https://github.com/MCUdude/MegaCoreX/blob/master/megaavr/bootloaders/optiboot/bootloaders/mega0/115200/Optiboot_mega0_UART0_DEF_115200_A7.hex

Download pyupdi from here:

https://github.com/mraardvark/pyupdi

Flash the bootloader using your FTDI board connected to the UPDI pin like this (see schematic below):

python.exe pyupdi.py -d mega4808 -c COM3 -f Optiboot_mega0_UART0_DEF_115200_A7.hex  -v -b 115200

Set BOOTEND fuse like this:

python.exe pyupdi.py -fs 8:0x02 -c COM3 -d mega4808

Set SYSCFG fuse like this:

python.exe pyupdi.py -fs 5:0xc9 -c COM3 -d mega4808

Set OSCFG fuse like this (I'm not sure if this was necessary)

python.exe pyupdi.py -fs 2:0x01 -c COM3 -d mega4808

In the above 4 lines, replace -d mega4808 with -d mega1608 if you're flashing an ATmega1608 with the bootloader.

Add this URL to your Additional board manager URLs in Arduino IDE and install the MegaCoreX boards in the Boards manager:

https://mcudude.github.io/MegaCoreX/package_MCUdude_MegaCoreX_index.json

Set the following board options in Arduino IDE:

• Board: ATmega4808
• Clock: Internal 8MHz
• BOD: BOD 2.6V
• Pinout: 32 pin standard
• Reset pin: Reset
• Bootloader: Optiboot (UART0 default pins)

Uploading Sketches and using NRF24L01+

Use the FTDI connector to upload sketches and use the serial monitor, like you would use it on a Arduino Pro Mini. After flashing the bootloader the UPDI pin isn't necessary anymore.

For the NRF24L01+ use the RF24 library by TMRh20: https://github.com/nRF24/RF24

Initialize it like this:

RF24 radio(9, 7);

The IRQ pin of the NRF24L01+ is connected to GPIO pin 10 on the ATmega.

Sources/more information

• https://www.microchip.com/wwwproducts/en/ATmega4808
• http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega4808-4809-Data-Sheet-DS40002173A.pdf
• http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega4809_Curiosity_Nano_Schematics.pdf
• http://ww1.microchip.com/downloads/en/appnotes/atmel-2521-avr-hardware-design-considerations_applicationnote_avr042.pdf
• https://github.com/MCUdude/MegaCoreX
• https://github.com/mraardvark/pyupdi
• https://oshwlab.com/wagiminator/atmega4808-development-board_copy