Datasheet

DocID022729 Rev 3 27/70
L6472 Functional description
70
6.8 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 the EXT_CLK and OSC_SEL parameters in the
CONFIG register (see Table 25 on page 50).
At power-up the device starts using the internal oscillator and provides a 2-MHz clock signal
on the OSCOUT pin.
Warning: In any case, before changing clock source configuration,
a hardware reset is mandatory. Switching to different clock
configurations during operation could 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
the OSC_SEL value); otherwise it is unused (see Figure 12).
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 12). The crystal/resonator and load capacitors (CL)
must be placed as close as possible to the pins. Refer to Tabl e 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 12).
Table 8. CL values according to external oscillator frequency
Crystal/resonator freq.
(1)
1. First harmonic resonance frequency.
CL
(2)
2. Lower ESR value allows the driving of greater load capacitors.
8 MHz 25 pF (ESR
max
= 80 )
16 MHz 18 pF (ESR
max
= 50 )
24 MHz 15 pF (ESR
max
= 40 )
32 MHz 10 pF (ESR
max
= 40 )