The RTC peripheral offers two timing functions: The Real-Time Counter (RTC) and a Periodic Interrupt Timer (PIT).

The PIT functionality can be enabled independently of the RTC functionality.

RTC - Real-Time Counter

The RTC counts (prescaled) clock cycles in a Counter register and compares the content of the Counter register to a Period register and a Compare register.

The RTC can generate both interrupts and events on compare match or overflow. It will generate a compare interrupt and/or event at the first count after the counter value equals the Compare register value, and an overflow interrupt and/or event at the first count after the counter value equals the Period register value. The overflow will reset the counter value to zero.

The RTC peripheral typically runs continuously, including in Low-Power sleep modes, to keep track of time. It can wake up the device from sleep modes, and/or interrupt the device at regular intervals.

The reference clock is typically the 32.768 kHz output from an external crystal. The RTC can also be clocked from an external clock signal, the 32.768 kHz internal ultra-low power oscillator (OSCULP32K), or the OSCULP32K divided by 32.

The RTC peripheral includes a 15-bit programmable prescaler that can scale down the reference clock before it reaches the counter. A wide range of resolutions and time-out periods can be configured for the RTC. 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 RTC also supports crystal error correction when operated using external crystal selection. An externally calibrated value will be used for correction. The software can adjust the RTC with an accuracy of ±1 PPM, and the maximum adjustment is ±127 PPM. The RTC correction operation will either speed up (by skipping count) or slow down (by adding extra count) the prescaler to account for the crystal error.

PIT - Periodic Interrupt Timer

The PIT uses the same clock source (CLK_RTC) as the RTC function and can generate an interrupt request or a level event on every nth clock period. The n can be selected from {4, 8, 16,... 32768} for interrupts, and from {64, 128, 256,... 8192} for events.