L9805 datasheet(14/103 Pages) STMICROELECTRONICS

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L9805

3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES

3.1 CLOCK SYSTEM

3.1.1 General Description

The MCU accepts either a Crystal or Ceramic res-

onator, or an external clock signal to drive the in-

ternal oscillator. The internal clock (fCPU) is de-

rived 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 fre-

quency 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 resona-

tor in the frequency range specified for fosc. The

circuit shown in Figure 5 is recommended when

using a crystal, and Table 2 lists the recommend-

ed 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 recom-

mended when crystals outside the specified fre-

quency 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 para-

sitic capacitance of the board and of the device).

Rp: External shunt resistance. Recommended val-

ue for oscillator stability is 1M

Ω.

Figure 4. External Clock Source Connections

Figure 5. Crystal/Ceramic Resonator

Figure 6. Clock Prescaler Block Diagram

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

OSCin

OSCout

COSCin

COSCout

OSCin

OSCout

COSCin

COSCout

%2,4,8,16

CPUCLK

to CPU and

Peripherals

to CAN

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