How an output port works, from flip-flop to PORTB

An output port is a register whose bits leave the processor and drive pins. An instruction writes it, and between writes it holds, so an LED stays lit while the program gets on with something else. A one-bit port is a D flip-flop with set and clear logic in front of it: D = ON OR (Q AND NOT OFF).

Set, keep, clear

ON is the decoder line of a "set" instruction, OFF the line of a "clear". On ON's tick, the OR makes D 1. On OFF's tick, the keep path is shut, so D is 0. On every other tick, a NOP or any instruction that isn't about the port, the keep path feeds Q back and the pin stays as it was.

A blink is a program

Put ON at address 0, OFF at address 2 and NOP at 1 and 3, and let the program counter run round. The LED is on for two ticks out of every four, for ever. Move OFF to address 3 and it's on for three; take OFF out altogether and, once ON has run, it never goes out. The hardware never changes. Only the program does.

On an Arduino: PORTB

The Uno's pins are bits of 8-bit port registers, and pin 13 is bit 5 of PORTB. digitalWrite(13, HIGH) ends up setting that one bit, after looking up which port and bit the pin is and checking for a timer on it. You can write the register yourself:

DDRB |= (1 << 5);    // pin 13 is an output
PORTB |= (1 << 5);   // set: LED on
PORTB &= ~(1 << 5);  // clear: LED off

The compiler turns each of the last two lines into a single instruction, SBI or CBI: set bit in I/O register, and clear it. They're this page's ON and OFF, and each takes two clock cycles. Writing the whole register at once also changes up to eight pins at the same instant, which digitalWrite can't do.

Faster than you can see

A processor can write a port millions of times a second. That's how it makes sounds on a speaker pin and dims LEDs with PWM: the same set and clear, repeated faster than an eye or ear can follow.

Ports are addresses

On many chips the ports sit in the same address space as memory, so an ordinary store instruction writes them: memory-mapped I/O. Every pin, screen, speaker and motor driver a processor controls sits behind a register like this one, holding its last instruction until the next.