GPIO

GPIO

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

GPIO Registers

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-func0tion-selection register, pullup-configuration register, etc).

INPUT HIGH IMPEDANCE (Hi-Z)

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.

Connecting switch 

Without internal PULLUP & PULLDOWN

Source & Sink current

• 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.

GPIO Internal Resistance

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 cautions 

GPIO Short circuit Scenarios:

GPIO Source Sink cautions (AVR ATmega328p)

How a Controller Decides HIGH or LOW on GPIO Input

Most of the controllers have a similar range as above.

Arduino

Functions

Blinking an LED void setup() {  pinMode(3, OUTPUT); } void loop() {  digitalWrite(3, HIGH);  delay(1000);  digitalWrite(3, LOW);  delay(1000); }

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); }