Pins D4, D13 and A5 on the Nano are reprogrammed to accept hardware interrupts.
Link to code:
https://akuzechie.blogspot.com/2020/1...
Contents:
0:00 Introduction
0:28 Arduino Nano PCI Pins
2:00 Programming PCI Registers
3:41 Circuit Description
4:16 Demonstration
5:09 C++ Sketch
5:51 Conclusion