Main Crystal Oscillator

The main crystal oscillator works with an external crystal, ceramic resonator, or a resistor-capacitor network to generate a high-precision clock in the range of 32 kHz to 20 MHz and is connected via the pins XTLOSC_[MAIN/AUX]_EXTAL and XTLOSC_[MAIN/AUX]_XTAL.

The following table lists the output frequency range of the main crystal oscillator with different possible sources.

Table 1. Crystal Oscillator Output Frequency Range


Output Frequency Range


32 kHz to 20 MHz

Ceramic Resonator

500 kHz to 4 MHz

R C Circuit

32 kHz to 4 MHz

The main crystal oscillator is operated in medium gain mode when a ceramic resonator is connected between the XTLOSC_[MAIN/AUX]_EXTAL and XTLOSC_[MAIN/AUX]_XTAL pins.

When a crystal is used, the load capacitance is determined by the external capacitors C1 and C2, internal capacitance, and stray capacitance (CS), as shown in the following figure.

Figure 1. Crystal Oscillator

The oscillator and the load capacitance should be used as per the recommendation of the manufacturer.

Table 2. Suggested Crystal Oscillator

CRYSTAL 32.768 kHz 12.5 pF SMD

Citizen CM519-32.768KEZF-UT

The frequency generated by an RC network is determined by the RC time constant of the selected components, as shown in the following figure.

Figure 2. RC Time Constant

The R and C components are connected to the XTLOSC_[MAIN/AUX]_EXTAL pin, with the XTLOSC_[MAIN/AUX]_XTAL pin connected to the power pin VPP, as shown in the following figure.

Figure 3. RC Oscillator

The operating mode of the main crystal oscillator is configured by the oscillator's macro available in Libero SoC.