ST7263BDX datasheet(26/145 Pages) STMICROELECTRONICS

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7.2 Clock system

7.2.1 General description

The MCU accepts either a crystal or ceramic reso-

nator, or an external clock signal to drive the inter-

nal oscillator. The internal clock (fCPU) is derived

from the external oscillator frequency (fOSC),

which is divided by 3 (and by 2 or 4 for USB, de-

pending on the external clock used). The internal

clock is further divided by 2 by setting the SMS bit

in the Miscellaneous Register.

Using the OSC24/12 bit in the option byte, a 12

MHz or a 24 MHz external clock can be used to

provide an internal frequency of either 2, 4 or 8

MHz while maintaining a 6 MHz for the USB (refer

to Figure 18).

The internal clock signal (fCPU) is also routed to

the on-chip peripherals. 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 or ceramic res-

onator in the frequency range specified for fosc.

The circuit shown in Figure 17 is recommended

when using a crystal, and Table 7, "Recommend-

ed values for 24 MHz crystal resonator" lists the

recommended capacitance. The crystal and asso-

ciated components should be mounted as close as

possible to the input pins in order to minimize out-

put distortion and start-up stabilisation time.

Table 7. Recommended values for 24 MHz

crystal resonator

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

7.2.2 External clock

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected, as

shown on Figure 16. The tOXOV specifications do

not apply when using an external clock input. The

equivalent specification of the external clock

source should be used instead of tOXOV (see Sec-

tion 6.5 CONTROL TIMING).

Figure 16. External clock source connections

Figure 17. Crystal/ceramic resonator

Figure 18. Clock block diagram

RSMAX

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

CPU and

8, 4 or 2 MHz

6 MHz (USB)

24 or

peripherals)

12 MHz

Crystal

OSC24/12

SMS

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