AN3326 datasheet(10/21 Pages) STMICROELECTRONICS

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Main oscillator

AN3326

10/21

DocID18333 Rev 3

Main oscillator

The SPC56EL60x uses an external crystal as input for both PLLs. The main oscillator

provides the following main features:



Input frequency range: 4–40 MHz



Crystal input mode



External reference clock (3.3 V) input mode



FMPLL reference

For noise immunity reasons, the oscillator uses a dedicated 3.3 V supply. This supply is

provided by the pins VDD_HV_OSC and VSS_HV_OSC (see Section 4).

Table 1 shows a list of the recommended crystals and their main parameters.

Note:

User shall configure the XOSC_MARGIN bit through the user option bit depending on the

chosen crystal.

XOSC_MARGIN must be set to ‘0’ if a 4 MHz crystal is chosen and set to ‘1’ in all other

cases.

3.1

Reference oscillator circuit

This section describes the key items of the oscillator circuit.

The oscillator circuit consists of the following components:



Crystal



Two capacitors

Figure 4 contains a schematic of the on-chip oscillator(b). Table 1 provides a list of

recommended crystals and the suggested value of the two load capacitors C1 and C2.

No other external components (for example, a bias resistor) are needed.

Table 1. Recommended crystals and their main parameters

Nominal frequency

[MHz]

Crystal model

Load on EXTAL/XTAL(1)

C1 = C2

[pF]

C1 and C2 include the parasitic capacitors which must be taken into account to choose the proper external

capacitors.

4.0

NX8045GB

4.7

8.0

NX5032GA

23.0

8.0

NX8045GB

23.0

10.0

NX5032GA

21.0

12.0

NX5032GA

19.0

16.0

NX5032GA

5.6

40.0

NX5032GA

8.0

40.0

NX3225GA

8.0

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