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I have problem with Arduino MEGA, Arduino Deu etc... Those Development board are having separate MCU except main MCU... Why? For what purpose are they used for?

as FTDI or bootloader chip or protection for main MCU or what other purpose?

I don't have an idea about that?

what are they called?

Please any one clarify me this?

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There are two different reasons for a second MCU on a development board.

1) If the main MCU has a bootloader for serial upload, but doesn't include USB peripheral and the dev board should have for convenient development an USB port, then the board must include a USB-to-TTL-Serial adapter ('FTDI'). The adapter can be an FTDI USB chip, some other manufacturers USB chip (most common is CH340) or an MCU (with a USB peripheral) programed as USB-to-TTL-Serial with a firmware. Original Arduinos use ATmega 8u2 or 16u2 for this.

2) Some dev boards include a programmer for upload without bootloader. The programmer is a second MCU running a programmer firmware and is wired to programming pins of the main MCU. For example Arduino Zero, M0 Pro or Uno WiFi Rev 2, Nano Every have this concept.

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