555-HV PS

An alternative to the ShiftRegister74HC595 library is the builtin shiftOut() function that is part of the Arduino IDE/libraries.

byte leds = 0;
bitSet(leds, i);

/*
 * updateShiftRegister() - This function sets the latchPin to low,
 * then calls the Arduino function 'shiftOut' to shift out contents
 * of variable 'leds' in the shift register before putting the 'latchPin' high again.
 */
void updateShiftRegister() {
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, leds);
    digitalWrite(latchPin, HIGH);
}

Faster pin reads. See https://forum.arduino.cc/t/digital-read-write-pinmode-fast-execution/89116

// --- PIN MODE: OUTPUT ---

DDRD |= _BV (0); // pinMode (0, OUTPUT); DDRD |= _BV (1); // pinMode (1, OUTPUT); DDRD |= _BV (2); // pinMode (2, OUTPUT); DDRD |= _BV (3); // pinMode (3, OUTPUT); DDRD |= _BV (4); // pinMode (4, OUTPUT); DDRD |= _BV (5); // pinMode (5, OUTPUT); DDRD |= _BV (6); // pinMode (6, OUTPUT); DDRD |= _BV (7); // pinMode (7, OUTPUT); DDRB |= _BV (0); // pinMode (8, OUTPUT); DDRB |= _BV (1); // pinMode (9, OUTPUT); DDRB |= _BV (2); // pinMode (10, OUTPUT); DDRB |= _BV (3); // pinMode (11, OUTPUT); DDRB |= _BV (4); // pinMode (12, OUTPUT); DDRB |= _BV (5); // pinMode (13, OUTPUT); DDRC |= _BV (0); // pinMode (A0, OUTPUT); DDRC |= _BV (1); // pinMode (A1, OUTPUT); DDRC |= _BV (2); // pinMode (A2, OUTPUT); DDRC |= _BV (3); // pinMode (A3, OUTPUT); DDRC |= _BV (4); // pinMode (A4, OUTPUT); DDRC |= _BV (5); // pinMode (A5, OUTPUT);

// --- PIN MODE: INPUT ---

DDRD &= ~_BV (0); // pinMode (0, INPUT); DDRD &= ~_BV (1); // pinMode (1, INPUT); DDRD &= ~_BV (2); // pinMode (2, INPUT); DDRD &= ~_BV (3); // pinMode (3, INPUT); DDRD &= ~_BV (4); // pinMode (4, INPUT); DDRD &= ~_BV (5); // pinMode (5, INPUT); DDRD &= ~_BV (6); // pinMode (6, INPUT); DDRD &= ~_BV (7); // pinMode (7, INPUT); DDRB &= ~_BV (0); // pinMode (8, INPUT); DDRB &= ~_BV (1); // pinMode (9, INPUT); DDRB &= ~_BV (2); // pinMode (10, INPUT); DDRB &= ~_BV (3); // pinMode (11, INPUT); DDRB &= ~_BV (4); // pinMode (12, INPUT); DDRB &= ~_BV (5); // pinMode (13, INPUT); DDRC &= ~_BV (0); // pinMode (A0, INPUT); DDRC &= ~_BV (1); // pinMode (A1, INPUT); DDRC &= ~_BV (2); // pinMode (A2, INPUT); DDRC &= ~_BV (3); // pinMode (A3, INPUT); DDRC &= ~_BV (4); // pinMode (A4, INPUT); DDRC &= ~_BV (5); // pinMode (A5, INPUT);

// --- DIGITAL WRITE: HIGH ---

PORTD |= _BV (0); // digitalWrite (0, HIGH); PORTD |= _BV (1); // digitalWrite (1, HIGH); PORTD |= _BV (2); // digitalWrite (2, HIGH); PORTD |= _BV (3); // digitalWrite (3, HIGH); PORTD |= _BV (4); // digitalWrite (4, HIGH); PORTD |= _BV (5); // digitalWrite (5, HIGH); PORTD |= _BV (6); // digitalWrite (6, HIGH); PORTD |= _BV (7); // digitalWrite (7, HIGH);

PORTB |= _BV (0); // digitalWrite (8, HIGH); PORTB |= _BV (1); // digitalWrite (9, HIGH); PORTB |= _BV (2); // digitalWrite (10, HIGH); PORTB |= _BV (3); // digitalWrite (11, HIGH); PORTB |= _BV (4); // digitalWrite (12, HIGH); PORTB |= _BV (5); // digitalWrite (13, HIGH);

PORTC |= _BV (0); // digitalWrite (A0, HIGH); PORTC |= _BV (1); // digitalWrite (A1, HIGH); PORTC |= _BV (2); // digitalWrite (A2, HIGH); PORTC |= _BV (3); // digitalWrite (A3, HIGH); PORTC |= _BV (4); // digitalWrite (A4, HIGH); PORTC |= _BV (5); // digitalWrite (A5, HIGH);

// --- DIGITAL WRITE: LOW ---

PORTD &= ~_BV (0); // digitalWrite (0, LOW); PORTD &= ~_BV (1); // digitalWrite (1, LOW); PORTD &= ~_BV (2); // digitalWrite (2, LOW); PORTD &= ~_BV (3); // digitalWrite (3, LOW); PORTD &= ~_BV (4); // digitalWrite (4, LOW); PORTD &= ~_BV (5); // digitalWrite (5, LOW); PORTD &= ~_BV (6); // digitalWrite (6, LOW); PORTD &= ~_BV (7); // digitalWrite (7, LOW);

PORTB &= ~_BV (0); // digitalWrite (8, LOW); PORTB &= ~_BV (1); // digitalWrite (9, LOW); PORTB &= ~_BV (2); // digitalWrite (10, LOW); PORTB &= ~_BV (3); // digitalWrite (11, LOW); PORTB &= ~_BV (4); // digitalWrite (12, LOW); PORTB &= ~_BV (5); // digitalWrite (13, LOW);

PORTC &= ~_BV (0); // digitalWrite (A0, LOW); PORTC &= ~_BV (1); // digitalWrite (A1, LOW); PORTC &= ~_BV (2); // digitalWrite (A2, LOW); PORTC &= ~_BV (3); // digitalWrite (A3, LOW); PORTC &= ~_BV (4); // digitalWrite (A4, LOW); PORTC &= ~_BV (5); // digitalWrite (A5, LOW);

// --- DIGITAL READ ---

x = (PIND & _BV (0)) == 0; // digitalRead (0); x = (PIND & _BV (1)) == 0; // digitalRead (1); x = (PIND & _BV (2)) == 0; // digitalRead (2); x = (PIND & _BV (3)) == 0; // digitalRead (3); x = (PIND & _BV (4)) == 0; // digitalRead (4); x = (PIND & _BV (5)) == 0; // digitalRead (5); x = (PIND & _BV (6)) == 0; // digitalRead (6); x = (PIND & _BV (7)) == 0; // digitalRead (7); x = (PINB & _BV (0)) == 0; // digitalRead (8); x = (PINB & _BV (1)) == 0; // digitalRead (9); x = (PINB & _BV (2)) == 0; // digitalRead (10); x = (PINB & _BV (3)) == 0; // digitalRead (11); x = (PINB & _BV (4)) == 0; // digitalRead (12); x = (PINB & _BV (5)) == 0; // digitalRead (13); x = (PINC & _BV (0)) == 0; // digitalRead (A0); x = (PINC & _BV (1)) == 0; // digitalRead (A1); x = (PINC & _BV (2)) == 0; // digitalRead (A2); x = (PINC & _BV (3)) == 0; // digitalRead (A3); x = (PINC & _BV (4)) == 0; // digitalRead (A4); x = (PINC & _BV (5)) == 0; // digitalRead (A5);