🛠 Apprivoiser les registres modernes de l'ATmega4809 : le timer TCB

Objectives

Leave the Arduino API behind and access registers directly: the ATmega4809 does NOT have the old T0/T1/T2 timers found on the 328P, but rather modern TCA and TCB peripherals. Here we configure a TCB (16-bit, periodic interrupt mode) that toggles LED 13 at 0.5 Hz without using delay or millis, by manipulating TCB0.CTRLA, TCB0.CCMP, TCB0.INTCTRL, and the INTFLAGS flag.

Hardware

• 1 × Arduino Nano Every
• Micro-USB cable
• Optional: logic analyzer on pin 13 to measure the period

Complete Code

// TCB0 en interruption periodique : toggle LED 13 sans delay
// Horloge noyau 20 MHz, diviseur /256 -> 78125 Hz
// CCMP = 78124 -> debordement 1 Hz -> LED basculee a 0,5 Hz
ISR(TCB0_OVF_vect) {
  TCB0.INTFLAGS = TCB_CAPT_bm;      // acquitter l'interruption
  digitalToggle(LED_BUILTIN);       // 13
}

void setup {
  pinMode(LED_BUILTIN, OUTPUT);

  TCB0.CTRLB = TCB_CNTMODE_INT_gc;              // periode simple
  TCB0.CCMP  = 78124;                           // 78125 / (78124+1) = 1 Hz
  TCB0.INTCTRL = TCB_CAPT_bm;                   // irq sur comparaison
  TCB0.CTRLA = TCB_CLKSEL_CLKDIV256_gc | TCB_ENABLE_bm;
  sei;                                        // autoriser les irq
}

void loop {
  // Le CPU ne fait rien : c'est le peripherique qui travaille.
}

Expected Verification

LED 13 blinks at 0.5 Hz (one toggle per second) while loop is empty. Remove the INTFLAGS acknowledgment: the LED stays lit (the interrupt re-triggers in a loop) — a pedagogical experiment on flags. Compare with tutorial 1: here the counting is done by hardware, and the application code remains 100% available.

Common Errors

• Looking for TCCR1B, OCR1A, TIMSK1: these registers do not exist on the 4809; the AVRex family uses TCA0/TCA1, TCB0 to TCB3, etc. Read the pinout diagram and the pins_arduino.h file of the nona4809 variant.
• Forgetting to acknowledge INTFLAGS in the ISR: permanent interrupt, the program seems frozen.
• Wrong clock divider: CLKSEL offers SYSCLK, /2, /16, /128, /256, or external clock — a CCMP calculated for /256 with /2 results in a frequency 128 times too high.
• Writing to the registers of a TCB already requisitioned by the core for PWM (analogWrite): check which timer serves which pin beforehand.
• sei forgotten: global interrupts remain masked, no ISR executes.