24.2 Overview

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 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 also 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 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 timeout periods can be up to two seconds. With a resolution of 1s, the maximum timeout period is more than 18 hours (65536 seconds). The RTC can give a compare interrupt and/or event when the counter equals the compare register value, and an overflow interrupt and/or event when it equals the period register value.

PIT - Periodic Interrupt Timer

Using the same clock source as the RTC function, the PIT can request an interrupt or trigger an output event on every nth clock period. n can be selected from {4, 8, 16,.. 32768} for interrupts, and from {64, 128, 256,... 8192} for events.

The PIT uses the same clock source (CLK_RTC) as the RTC function.