L9803 datasheet(18/126 Pages) STMICROELECTRONICS

18 / 126 page

Clocks, Reset, Interrupts & Power saving modes

L9803

18/126

Clocks, Reset, Interrupts & Power saving modes

3.1

Clock system

3.1.1

General Description

The MCU accepts either a Crystal or Ceramic resonator, or an external clock signal to drive

the internal oscillator. The internal clock (fCPU) is derived from the external oscillator

frequency (fOSC). The external Oscillator clock is first divided by 2, and an additional division

factor of 2, 4, 8, or 16 can be applied, in Slow Mode, to reduce the frequency of the fCPU;

this clock signal is also routed to the on-chip peripherals (except the CAN). The CPU clock

signal consists of a square wave with a duty cycle of 50%.

The internal oscillator is designed to operate with an AT-cut parallel resonant quartz crystal

resonator in the frequency range specified for fosc. The circuit shown in Figure 6 is

recommended when using a crystal, and Table 2 lists the recommended capacitance and

feedback resistance values. The crystal and associated components should be mounted as

close as possible to the input pins in order to minimize output distortion and start-up

stabilisation time.

Use of an external CMOS oscillator is recommended when crystals outside the specified

frequency ranges are to be used.

Table 2.

Recommended Values for 16 MHz Crystal Resonator

Note:

RSMAX is the equivalent serial resistor of the crystal (see crystal specification).

COSCIN,COSCOUT: Maximum total capacitances on pins OSCIN and OSCOUT (the value

includes the external capacitance tied to the pin plus the parasitic capacitance of the board

and of the device).

Rp: External shunt resistance. Recommended value for oscillator stability is 1M

Ω.

Figure 5.

External Clock Source Connections

RSMAX

Ω

150

Ω

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

1-10 M

Ω

1-10 M

Ω

1-10 M

Ω

OSCin

OSCout

EXTERNAL

CLOCK

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