ST7262XXX datasheet(20/139 Pages) STMICROELECTRONICS

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ST7262xxx

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Doc ID 6996 Rev 5

6 CLOCKS AND RESET

6.1 CLOCK SYSTEM

6.1.1 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),

by dividing by 3 and multiplying by 2. By setting the

OSC12/6 bit in the option byte, a 12 MHz external

clock can be used giving an internal frequency of 8

MHz while maintaining a 6 MHz clock for USB (re-

fer to Figure 14).

The internal clock signal (fCPU) consists of a

square wave with a duty cycle of 50%.

It is further divided by 1, 2, 4 or 8 depending on the

Slow Mode Selection bits in the Miscellaneous

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 13 is recommended

when using a crystal, and Table 4 lists the recom-

mended capacitors. The crystal and associated

components should be mounted as close as pos-

sible to the input pins in order to minimize output

distortion and start-up stabilization time.

Table 4. Recommended Values for 12 MHz

Crystal Resonator

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

Note: When a crystal is used, and to not over-

stress the crystal, ST recommends to add a serial

resistor on the OSCOUT pin to limit the drive level

in accordance with the crystal manufacturer’s

specification. Please also refer to Section 12.5.4.

6.1.2 External Clock input

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected, as

shown on Figure 12. The tOXOV specifications

does not apply when using an external clock input.

The equivalent specification of the external clock

source should be used instead of tOXOV (see Elec-

trical Characteristics).

6.1.3 Clock Output Pin (MCO)

The internal clock (fCPU) can be output on Port B0

by setting the MCO bit in the Miscellaneous regis-

ter.

Figure 12. External Clock Source Connections

Figure 13. Crystal/Ceramic Resonator

RSMAX

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

1-10 M

OSCIN

OSCOUT

EXTERNAL

CLOCK

OSCIN

OSCOUT

COSCIN

COSCOUT

Obsolete

Product(s)

- Obsolete

Product(s)

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