L6480 datasheet(26/73 Pages) STMICROELECTRONICS

26 / 73 page

Functional description

L6480

26/73

Doc ID 023278 Rev 3

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

CONFIG register (see Table 32).

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.

6.8.1

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 11).

6.8.2

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 11). The crystal/resonator and load capacitors (CL)

must be placed as close as possible to the pins. Refer to Table 8 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 11).

The L6480 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 GATECFG1 register

(Section 9.1.21). When the external clock source is selected, the device continues to work

with the integrated oscillator for textosc milliseconds and then the clock management system

switches to the OSCIN input.

Table 8.

CL values according to external oscillator frequency

Crystal/resonator freq. (1)

First harmonic resonance frequency.

(2)

Lower ESR value allows driving greater load capacitors.

8 MHz

25 pF (ESRmax = 80 Ω)

16 MHz

18 pF (ESRmax = 50 Ω)

24 MHz

15 pF (ESRmax = 40 Ω)

32 MHz

10 pF (ESRmax = 40 Ω)

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