Optimizing the reaction time of the ATMega328

classical-approach.cpp

/**
 * ---------------------------------------------------------------------------
 * Control of an LED by a push button with an Arduino Nano board
 * ---------------------------------------------------------------------------
 * Classical approach with the Arduino API
 * ---------------------------------------------------------------------------
 * PIN |
 * ----+----------------------------------------------------------------------
 * D2  | Push button wired up to a pull-down resistor (10 kΩ)
 * ----+----------------------------------------------------------------------
 * D4  | Red LED with a 220 Ω resistor in series
 * ----+----------------------------------------------------------------------
 */

#include <Arduino.h>

#define BTN_PIN 2
#define LED_PIN 4
#define DEBOUNCE_DELAY_MS 1

bool     led_level = LOW;
uint8_t  input;
uint8_t  last_input;
uint8_t  output;
uint32_t last_debounce;

void setup() {
    pinMode(BTN_PIN, INPUT);
    pinMode(LED_PIN, OUTPUT);
}

void loop() {

    uint32_t now = millis();

    input = digitalRead(BTN_PIN);

    if (input != last_input) last_debounce = now;
    last_input = input;

    if (now - last_debounce > DEBOUNCE_DELAY_MS) {
        if (output != input) {
            output = input;
            if (output) {
                digitalWrite(LED_PIN, led_level = !led_level);
            }
        }
    }

}

optimized-approach.cpp

/**
 * ---------------------------------------------------------------------------
 * Control of an LED by a push button with an Arduino Nano board
 * ---------------------------------------------------------------------------
 * Dramatically improved reaction time of the ATMega328
 * ---------------------------------------------------------------------------
 * Pinout:
 * ----+------+
 * PIN | PORT |
 * ----+------+---------------------------------------------------------------
 * D2  | PD2  | Push button wired up to a pull-down resistor (10 kΩ)
 * ----+------+---------------------------------------------------------------
 * D4  | PD4  | Red LED with a 220 Ω resistor in series
 * ---------------------------------------------------------------------------
 * Instead of using the digitalRead() and digitalWrite() functions of the
 * Arduino API, we will instead use the internal registers, and the INT0
 * interrupt vector (External Interrupt Request 0) of the ATMega328.
 * ---------------------------------------------------------------------------
 * As a result, the reaction time of the ATMega328 is 1000 times faster!
 * ---------------------------------------------------------------------------
 * Documentation:
 * 
 * @see https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf
 * 
 * I/O Ports:
 * 
 * PORTD - The Port D Data Register
 * ----+--------+-------+-----------------------------+---------
 * PD2 | PORTD2 | Bit 2 | External Interrupt Source   | pp.71,73
 * PD4 | PORTD4 | Bit 4 | Pin Change Interrupt Source | pp.70,73
 * ----+--------+-------+-----------------------------+---------
 * 
 * Register Description:
 * -------+---------------------------------------+-------+-----
 * SREG   | AVR Status Register                   |       | p.11
 * SREG_I | Global Interrupt Enable               | Bit 7 | p.11
 * EICRA  | External Interrupt Control Register A |       | p.54
 * ISC01  | Interrupt Sense Control 0             | Bit 1 | p.54
 * ISC00  | Interrupt Sense Control 0             | Bit 0 | p.54
 * EIMSK  | External Interrupt Mask Register      |       | p.55
 * INT0   | External Interrupt Request 0 Enable   | Bit 0 | p.55
 * EIFR   | External Interrupt Flag Register      |       | p.55
 * INTF0  | External Interrupt Flag 0             | Bit 0 | p.55
 * DDRD   | Port D Data Direction Register        |       | p.73
 * -------+---------------------------------------+-------+-----
 * 
 * Interrupt Sense Control 0:
 * ------+-------+------------------------------------------------------------
 * ISC01 | ISC00 | Description
 * ------+-------+------------------------------------------------------------
 *   0   |   0   | The low level of INT0 generates an interrupt request
 *   0   |   1   | Any logical change on INT0 generates an interrupt request
 *   1   |   0   | The falling edge of INT0 generates an interrupt request
 *   1   |   1   | The rising edge of INT0 generates an interrupt request
 * ------+-------+------------------------------------------------------------
 */

#include <Arduino.h>

#define DEBOUNCE_DELAY_MS 1

ISR(INT0_vect) {

    EIMSK &= ~(1 << INT0); // Override of interrupt vector INT0
    PORTD ^= 1 << PD4;     // PD4 bit toggle

}

void setup() {

    DDRD  &= ~(1 << PD2);                 // Setting the button reading pin
    DDRD  |= 1 << PD4;                    // Setting the LED control pin
    EICRA  = (1 << ISC01) | (1 << ISC00); // Setting the interrupt vector INT0 (RISING)
    EIMSK  = 1 << INT0;                   // Arming the interrupt vector INT0
    SREG  |= 1 << SREG_I;                 // Activation of interrupts
    
}

void loop() {

    static uint32_t last = 0;
           uint32_t now  = millis();

    // If the interrupt vector INT0 has been disarmed,
    // and the debouncing time has expired...
    if (!(EIMSK >> INT0) && now - last > DEBOUNCE_DELAY_MS) {

        last = now; // Reset the stabilization window

        // Resets the interrupt vector INT0
        // as soon as the read pin is at LOW
        if (!((PIND >> PD2) & 0x1)) {
            EIFR  = 1 << INTF0; // Cancels any previous interruption on interrupt vector INT0
            EIMSK = 1 << INT0;  // Rearming the interrupt vector INT0
        }

    }

}