Why are pin numbers on Arduino Nano jumbled?

I'm looking at a schematic of the pinout of the Arduino Nano and I see that the sequence starts:

31, 30, 29, GND, 32, 1, 2, 9, 10...

Why not:

1, 2, 3, GND, 4, 5, 6, 7, 8... ?


Can you show us the schematic in question?

I can't tell what you're referring to.

I agree. Looking over the V3 Nano schematic & board layout, I don't see what the OP is describing.

Presumably, the upper left section this picture (not really a schematic) of the common Nano pinout...

there are a couple of reasons for this:

  • Let's be honest: these "tiny" boards are laid out in a way that makes the PCB routing traces "convenient" without needing too many layers, vias, or long thin convoluted traces. By the time you place the USB and ISP connectors, center the reset switch, and lump the LEDs in one place, you've eliminated a lot of options. Assuming certain package types/etc.
  • The numbers you're talking about are the chip pin numbers, which are somewhat irrelevant in a module, and especially so within the arduino environment.
  • It somewhat preserves the "logical grouping" of the digital and analog pins. D0...D13 "together", A0..A5, etc.
  • it puts some signals conveniently near the 'end' of the board. (Notably RX/TX)

For a more recent board design with smaller components and updated manufacturing tech (?), see something like the Adafruit Metro Mini. (But the Nano is such a widely-produced board (in multiple varieties) that the old pinout isn't likely to ever go away...

Those are the physical pin numbers - what matters is the logical pin numbers, which are in a more reasonable order.