POWERSTEP01 datasheet(29/92 Pages) STMICROELECTRONICS

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powerSTEP01

Functional description

7.8

Internal oscillator and oscillator driver

The control logic clock can be supplied by the internal 16-MHz oscillator, an external

oscillator (crystal or ceramic resonator) or a direct clock signal.

These working modes can be selected by EXT_CLK and OSC_SEL parameters in the

CONFIG register (see Table 41 on page 66).

At power-up the device starts using the internal oscillator and provides a 2-MHz clock signal

on the OSCOUT pin.

Attention: In any case, before changing clock source configuration,

a hardware reset is mandatory. Switching to different clock

configurations during operation may cause unexpected

behavior.

7.8.1

Internal oscillator

In this mode the internal oscillator is activated and OSCIN is unused. If the OSCOUT clock

source is enabled, the OSCOUT pin provides a 2, 4, 8 or 16-MHz clock signal (according to

OSC_SEL value); otherwise it is unused (see Figure 12).

7.8.2

External clock source

Two types of external clock source can be selected: crystal/ceramic resonator or direct clock

source. Four programmable clock frequencies are available for each external clock source:

8, 16, 24 and 32-MHz.

When an external crystal/resonator is selected, the OSCIN and OSCOUT pins are used to

drive the crystal/resonator (see Figure 12). The crystal/resonator and load capacitors (CL)

must be placed as close as possible to the pins. Refer to Table 7 for the choice of the load

capacitor value according to the external oscillator frequency.

If a direct clock source is used, it must be connected to the OSCIN pin and the OSCOUT pin

supplies the inverted OSCIN signal (see Figure 12).

The powerSTEP01 integrates a clock detection system that resets the device in case of the

failure of the external clock source (direct or crystal/resonator). The monitoring of the clock

source is disabled by default, it can be enabled setting high the WD_EN bit in the

Table 7. CL values according to external oscillator frequency

Crystal/resonator freq.(1)

1. First harmonic resonance frequency.

CL(2)

2. Lower ESR value allows driving greater load capacitors.

8 MHz

25 pF (ESRmax = 80 )

16 MHz

18 pF (ESRmax = 50 )

24 MHz

15 pF (ESRmax = 40 )

32 MHz

10 pF (ESRmax = 40 )

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