2.6 RTC - Real-Time Counter

The Real-Time Counter (RTC) is a peripheral that typically runs continuously, including in Low-Power sleep modes, to keep track of time. It offers two timing functions: The Real-Time Counter (RTC) and a Periodic Interrupt Timer (PIT). It can wake up the device from sleep modes and/or interrupt the device at regular intervals.

The RTC peripheral works in Idle and Standby sleep modes. A wide range of resolutions and time-out periods can be configured for it. With a 32.768 kHz clock source, the maximum resolution is 30.5 μs, and time-out periods can be up to two seconds. With a resolution of 1s, the maximum time-out period is more than 18 hours (65536 seconds).

The following code snippets show the initialization of RTC to generate interrupts with a 1-second periodicity (the interrupt code is not included):

void RTC_0_init()
{
    while (RTC.STATUS > 0) {   /* Wait for all register to be synchronized */
    }
    RTC.PER = 0x3FF;           /* Period: 0x3FF - 1024 RTC clock cycles */
    RTC.CLKSEL = RTC_CLKSEL_OSC1K_gc;  /* 32kHz divided by 32 */
    RTC.INTCTRL = 0 << RTC_CMP_bp   /* Compare Match Interrupt enable: disabled */
                | 1 << RTC_OVF_bp;   /* Overflow Interrupt enable: enabled */
}

In addition to the RTC functionality, the RTC of the AVR Dx devices includes a Periodic Interrupt Timer (PIT) that can run even in Power-Down mode. If enabled, it can generate an interrupt request on every 2ⁿ clock period, where n can have any value from 2 to 15, selectable via the RTC.PITCTRLA register. This mode allows periodic interrupts/wake-up from sleep when the device is in Power-Down mode.