Arduino Uno R3

Fig. 1. Arduino Uno R3, Uno shield form factor, 68.6 x 53.4 mm. Hover or tap a signal to light up every pin that carries it.

• 20 mA per I/O pin is the design figure; 40 mA is the absolute maximum and 200 mA is the limit across the whole chip. Drive motors and strips through a transistor or driver.
• D13 is wired to the on-board LED through a resistor, so it already has a load. It is a poor choice for an input.
• A4/A5 are the only I2C pins. A0-A5 all work as ordinary digital pins (14-19) when you do not need the ADC.
• D0/D1 are shared with the USB-serial bridge. Anything attached there fights the serial monitor and the bootloader.
• VIN accepts 7-12 V through the barrel jack regulator. Feeding 5 V into the 5V pin bypasses the regulator and its protection.
• The R3 and R4 have the same header layout, but the R4 is a 5 V RA4M1 (Minima) or RA4M1 + ESP32-S3 (WiFi) and its peripheral mapping differs.

• 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.