Main oscillator

AN2871

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Doc ID 15304 Rev 3

3

Main oscillator

SPC560Pxx devices can run with an external oscillator used as input for the PLLs, and

selected also as the system clock source. The main oscillator provides these features:

●

Frequency range: 4–40 MHz

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Crystal input mode or Oscillator input mode

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PLL reference

●

Oscillator supply: for noise immunity reasons, the oscillator supply uses dedicated

3.1

Reference oscillator circuit

Figure 5 provides a schematic of the on-chip oscillator. This section describes the key items.

Figure 5.

Reference oscillator circuit

The oscillator circuit provides a reference clock signal to the on-chip PLL. The oscillator

circuit consists of the following components:

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Bias resistor (R1)

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Crystal

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Two capacitors

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External bias resistor (R2)

Note:

The external resistor (R2) is not recommended due to the fact that the oscillator has an

internal bias resistor. However, it is recommended to leave room for an external bias resistor

to allow the PCB design to accommodate different crystals.

Oscillator hardware recommendations:

●

Use the lowest frequency crystal possible and set the multiplication factor bits to obtain

the proper system operating frequency which is generated from the PLL.

●

The oscillator circuit has currents flowing at the crystal’s fundamental frequency. Also, if

the oscillator is clipped, then higher order harmonics will be present as well. In order to

minimize the amount of emissions generated from these currents, the oscillator circuit

should be kept as compact as possible.

R2 = 1 M



R1 = 0



C1 = 22 pF

XTAL

EXTAL

SPC560Pxx

C2 = 22 pF

XTAL = 8 MHz

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