Quick query (I hope).

I have an Arduio R3 board with ATMega328P U on board. It works fine. I bought some new MCUs, ordered with bootloader. Same IC designation. Before I dive into my new project I want to confirm they are okay.

So ...............I take the original MCU out of the Arduino R3 and replace it with a new ATMega328P U. Should it work fine straight away? i.e. can I upload simple sketches like Blink?

My problem is that I've done this and none of them work. I get two flashes on the Rx LED on the board, then ArvDude gives me a string of "not in sync" error messages. If I put the original MCU back in then everything works okay again. I don't want to go through the hassle of burning bootloader on chips that supposedly have it, I'd rather send them back.

Are my new MCUs duff? Or is there any other easy way to check them?

Many thanks

  • 1
    An atmega328p "with bootloader" might have something different than the current optiboot preloaded. Choose a "Nano with old bootloader" as your arduino target. Jul 14, 2020 at 14:28

1 Answer 1


The MCU on the Uno R3 comes with a specific bootloader (Optiboot) and fuse configuration (external clock, clock prescaler disabled, bootloader enabled...). You may find some ATmega328Ps on the market that have been specifically configured as drop-in replacements for the one on an Uno. Those should “just work”. If the ones you bought come with a different bootloader or fuse configuration, you may have to re-flash or reconfigure them.

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