External Frequency Measurement

When using an external measurement tool to do frequency measurement, the 32.768 kHz crystal oscillator output must be available on a pin. This can be done either using an Event from the RTC or PIT, and configuring a pin as a user, or by using the 32.768 kHz crystal as the main clock.

The RTC, when enabled, will generate the following output events:

Overflow (OVF): Generated when the counter has reached its top value and wrapped to zero. The generated strobe is synchronous with CLK_RTC and lasts one CLK_RTC cycle.
Compare (CMP): Indicates a match between the counter value and the Compare register. The generated strobe is synchronous with CLK_RTC and lasts one CLK_RTC cycle.

When enabled, the PIT generates the following 50% duty cycle clock signals on its event outputs:

Event 0: Clock period = 8192 RTC clock cycles
Event 1: Clock period = 4096 RTC clock cycles
Event 2: Clock period = 2048 RTC clock cycles
Event 3: Clock period = 1024 RTC clock cycles
Event 4: Clock period = 512 RTC clock cycles
Event 5: Clock period = 256 RTC clock cycles
Event 6: Clock period = 128 RTC clock cycles
Event 7: Clock period = 64 RTC clock cycles

The event users are configured by the Event System (EVSYS).

When using the 32.768 kHz crystal as the main clock source, there are several ways of routing the clock out of the pin.

Routing the clock directly to the CLKOUT pin.
Using TCA, TCB, TCD or SPI to output the main clock frequency.

It is not recommended to attach a probe directly to the crystal circuit, to measure the frequency. This will change the characteristics of the circuit and alter the crystal frequency.

The following two examples shows how to output the 32.768 kHz crystal to a pin. One example shows how to use the system clock, and one shows how to use the RTC and Event System.