1.4.1 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 1-12. Crystal Oscillator Output Frequency Range

Source

Output Frequency Range

Crystal

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 1-4. Crystal Oscillator

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

Table 1-13. Suggested Crystal Oscillator

CRYSTAL 32.768 kHz 12.5 pF SMD

CitizenCM519-32.768KEZF-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 1-5. 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 1-6. RC Oscillator

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