Recently I've seen a Arduino Micro schematic available on site:
and I don't understand some things there.
- There is a 10K resistor and 100nF cap connected to VUSB. What function have a 10K resistor there?
- Why AVCC1 pin (pin 44) is connected via ferrite bead, and AVCC (pin 23) is not?
Thanks a lot for answer!
Well the 100nF decoupling isn’t surprizing, the 10k means the rail will discharge in finite time if unplugged.
The lack of ferrite bead on one of the AVcc’s is a mystery.
Pin 44 is the main power input for the analogue unit which needs the LC filter provided by the ferrite and capacitor; the datasheet does not seem to mention the use of pin 23.
However the datasheet does state :-
"The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ± 0.3V from VCC"
So suspect pin23 is used to provide Vcc to the part of the analogue circuitry that that compares the two rails;
probably less noise / problematic to use an external pin rather than across the chip... ?