Why do some boards have two processors?

[markd833]

@jchan5

If all the SAMD11 is doing on the Nano is acting like a USB-Serial interface, it kind of feels like it's being underutilized. Could there be another reason for this dual architecture?

I believe that the SAMD51 is also a programmer for the 4809. The 4809 can programmed using the UPDI (Unified Program and Debug Interface).

You can build a UPDI programmer using an Arduino UNO (or Nano etc) in order to get a bootloader into the 4809. Have a read of this: Minimal ATmega4809 on a Breadboard.