The ACT module requires the use of a 32.768 kHz external crystal connected to the SOSC pins. The SOSC time base is then used to adjust the frequency of the HFINTOSC over the datasheet specified voltage and temperature range of the device. The ACT module does this by actively adjusting the OSCTUNE<5:0> bits in real time to closely maintain the calibrated oscillator frequency.
When the ACT module is enabled, the OSCTUNE bits are read-only to the user. Any writes by the user when the module is enabled are inhibited. The ACT module will adjust the HFINTOSC to maintain the calibrated frequency by updating the OSCTUNE bits at the specified ACT update rate. The operation of the ACT module works without the need for user software interaction, and instead only relies on the initial module configuration.
The ACT module makes the internal HFINTOSC a high-accuracy oscillator by using the SOSC as a “fixed” reference source over voltage and temperature. The ACT will only be as accurate as the 32.768 kHz crystal used in the application. It is important to note that if the selected crystal varies considerably over voltage or temperature, HFINTOSC will also display the same behavior.