Arduino Nano v3

Fig. 1. Arduino Nano v3, 2x15 2.54 mm DIL, 18 x 45 mm. Hover or tap a signal to light up every pin that carries it.

• A6 and A7 are analog inputs only. They have no digital driver, so digitalRead/digitalWrite do nothing on them, a surprise that costs people an afternoon.
• A4/A5 are the only I2C pins; D10-D13 are the SPI pins.
• VIN takes 7-12 V. The regulator is small, so a 12 V supply plus a few peripherals will get hot.
• Clones ship with a CH340 instead of the original FT232RL, which needs a different USB driver but is otherwise interchangeable.
• RESET appears twice on the header, which is handy when the ICSP side is occupied.
• The newer Nano Every, Nano 33 IoT and Nano ESP32 keep this footprint but are 3.3 V parts with different peripherals.

• Drive a pin, and read one with a pull-up

  avr-libc

  #include <avr/io.h>
  
  /* Three registers per port. DDR picks direction, PORT writes the
     output latch, PIN reads the physical pin. */
  
  DDRB  |=  (1 << PB5);          /* PB5 = Arduino D13 = output */
  PORTB |=  (1 << PB5);          /* high */
  PORTB &= ~(1 << PB5);          /* low  */
  PINB  |=  (1 << PB5);          /* writing PIN toggles the output */
  
  DDRD  &= ~(1 << PD2);          /* PD2 = D2 = input */
  PORTD |=  (1 << PD2);          /* PORT on an input enables the pull-up */
  
  if (!(PIND & (1 << PD2))) {    /* switch to ground reads 0 when pressed */
  }

  On an input pin, PORT is the pull-up enable, not an output value. Forget it and a floating pin reads whatever the nearest mains cable is doing.

Table 1. Pin assignments.