ST7263BEx datasheet(35/186 Pages) STMICROELECTRONICS

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ST7263Bxx

Reset and clock management

Doc ID 7516 Rev 8

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6.2

Clock system

6.2.1

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), which is divided by 3 (and by 2 or 4 for USB, depending 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 resonator in the frequency range specified for fosc. The circuit shown in Figure 17 is

recommended when using a crystal, and Table 8 lists the recommended capacitance. 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 stabilization time.

6.2.2

External clock

An external clock may be applied to the OSCIN input 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 Table 62: Control timing characteristics).

Figure 16.

External clock source connections

Table 8.

Recommended Values for 24 MHz crystal resonator

Recommended capacitance and resistance

RSMAX

(1)

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

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

1-10 M

OSCIN

OSCOUT

EXTERNAL

CLOCK

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