🛠 Manipulation directe des registres PORT : le bit-banging à 16 MHz

Objectives

digitalWrite is convenient but slow on an 8-bit AVR: about ten cycles per call, limiting the pulse width to a few tens of kHz. To drive neopixels, multiplexed displays, or a custom protocol, you drop down to the register level: DDRx, PORTx, PINx. The official Arduino pin ↔ port bit mapping table (PinMapping168) is your navigation map. You will generate a pulse measurable with an oscilloscope and write to an entire port in a single write operation.

Hardware

• 1 Arduino Duemilanove (ATmega328)
• 1 oscilloscope or logic analyzer (otherwise, a frequency counter on the pin)
• No wiring required: the LED on pin 13 serves as a visible load

Complete code

// Carre rapide sur la broche 13 = PORTB bit 5 (voir PinMapping168)
// Broches 8..13 = PORTB bits 0..5 ; broches 0..7 = PORTD bits 0..7 ;
// broches analogiques A0..A5 = PORTC bits 0..5.

void setup {
  DDRB |= (1 << DDB5);      // broche 13 en sortie (registre DDR du port B)
  // Equivalence : pinMode(13, OUTPUT);
}

void loop {
  noInterrupts;           // le timer0 du millis perturbe les mesures
  PORTB |=  (1 << PORTB5);  // broche 13 a 1  -> 1 cycle CPU
  PORTB &= ~(1 << PORTB5);  // broche 13 a 0  -> 1 cycle CPU
  // NB : |= et &= font 2 ecritures ; en assembleur, sbi/cbi font 1 cycle.
  interrupts;
}

Writing to 8 LEDs at once: PORTD = B10101010; sets pins 0–7 to the desired state in a single instruction (note: pins 0/1 are the FTDI serial link — do not break the monitor during uploading).

Expected verification

On the oscilloscope connected to pin 13, you should see a pulse with a period measured in hundreds of nanoseconds (C loops run at around ~1–2 MHz switching speed, much faster with sbi/cbi in assembly) — compared to ~50 kHz maximum with digitalWrite. LED 13 remains dimly lit (natural PWM due to duty cycle). The "one write, 8 LEDs" test toggles an entire port simultaneously.

Common errors

• Writing to PORTD without thinking: bits 0 and 1 are RX/TX to the FTDI; an inadvertent PORTD = 0 breaks the serial monitor and can interfere with uploading.
• Forgetting DDRx: writing to PORTx without setting the pin as output controls nothing (or just activates the internal pull-up, PORTx=1 while in input mode).
• Confusing PORTx and PINx: reading the actual pin state is done via PINx; reading PORTx reads back the output setting.
• Modifying a shared port via read-modify-write inside an interrupt: risk of race condition with the main code; use atomic bit operations (sbi/cbi) or disable interrupts.
• Forgetting that millis runs via timer0: the 1 ms interrupts add jitter to frequency measurements — hence noInterrupts during measurement.
• Using registers on an ATmega168 chip assuming the same pins: the mapping is identical for standard pins, but the SRAM (1 KB) and flash (14,336 bytes) quickly limit large lookup tables.
• Verifying with a multimeter: at these frequencies, it displays a misleading average voltage — you need an oscilloscope.