4. GPIO Usage and Applications

• General-Purpose Input Output (GPIO) used for INPUT and OUTPUT.
  • GPIO INPUT → switch, buttons, keypad.
  • GPIO OUTPUT → LED, Relay, 7-segment display.

Generally, in controllers (AVR, PIC) following types of GPIO registers are present.

Considering the AVR controller

where x = port number (eg B,C,D) 

NOTE: ARM and RISC-based 32-bit controllers have more add-on registers (e.g. PIN-function-selection register, pullup-configuration register, etc).

It's like an open circuit — the pin 'floats' and its voltage can randomly vary due to electrical noise, making it unreliable unless we connect external voltage to it.

Without internal PULLUP & PULLDOWN

With internal PULLUP & PULLDOWN

• Source: The current flows from the GPIO pin to the load. (e.g. GPIO HIGH powering an LED).
• Sink: The current flows from the load into the GPIO pin (which acts like ground). (e.g. LED cathode (- ve) connected to GPIO).
   

Max Limits: Each microcontroller has max source/sink current capacity per pin. Exceeding it can damage the pin or controller.

E.g. – ATmega328 has a max GPIO source / sink current limit of 40 mA. Although it is recommended to keep it minimum (< 20 mA).

GPIO Port’s current limit:
Controller’s PORT also has its own limits for source/sink current.

E.g., AVR ATmega328 PORTC has a max sink current limit of 100 mA. As it has only 3 PORTs, the max source/sink current it can handle is up to 300 mA.

Controller's GPIO has some internal resistance e.g., ~25Ω for AVR ATmega328.
So when OUTPUT is HIGH and current flows, it drops the GPIO voltage.

GPIO Short Circuit Scenarios:

GPIO Source Sink cautions (AVR ATmega328p)

Most of the controllers have a similar range as above.

1. Blinking an LED

void setup() {
  pinMode(3, OUTPUT);
}

void loop() {
  digitalWrite(3, HIGH);
  delay(1000);
  digitalWrite(3, LOW);
  delay(1000);
}

2. Control LED with a Switch

int pushButton = 2;
int LED = 7;

void setup() {
  pinMode(pushButton, INPUT_PULLUP);
  pinMode(LED, OUTPUT);
}

void loop() {
  int buttonState = digitalRead(pushButton);
  digitalWrite(LED, !buttonState);
  delay(1);
}