Active Clock Tuning

Active Clock Tuning (ACT) is a feature on some 8-bit PIC® MCU devices. The Active Clock Tuning (ACT) module continuously adjusts the 16 MHz Internal Oscillator, using an available external reference, to achieve ± 0.20% accuracy. This eliminates the need for a high-speed, high-accuracy external crystal when the system has an available lower speed, lower power, high-accuracy clock source available such as the High Frequency Internal Oscillator (HFINTOSC).

Systems implementing a Real-Time Clock Calendar (RTCC) or a full-speed USB application can take full advantage of the ACT.


The HFINTOSC is tuned using Full-speed USB events or the 32.768 kHz oscillator (SOSC) clock source. The ACT is enabled by setting the ACTEN bit of the ACTCON register. When enabled, the ACT takes control of the OSCTUNE register. The ACT uses the
selected ACT reference clock to tune the 16 MHz Internal Oscillator to an accuracy of 16 MHz ± 0.2%.
The tuning automatically adjusts the OSCTUNE register every reference clock cycle.


The ACT reference clock is selected with the ACTSRC bit of the ACTCON register. The reference clock sources are provided by the:

  • USB module in full-speed operation (ACT_clk)
  • Secondary clock at 32.768 kHz (SOSC_clk)
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