ST72611F1M1 データシートの表示（PDF） - STMicroelectronics

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ST72611F1M1

LOW SPEED USB 8-BIT MCU WITH 3 ENDPOINTS, FLASH MEMORY, LVD, WDG, TIMER

STMicroelectronics 

ST7261

5.2 CLOCK SYSTEM

5.2.1 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 17).

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

register (SMS[1:0])

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 16 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.

Figure 15. External Clock Source Connections

OSCIN

OSCOUT

NC

EXTERNAL

CLOCK

Figure 16. Crystal/Ceramic Resonator

OSCIN OSCOUT

COSCIN

COSCOUT

Table 4. Recommended Values for 12 MHz

Crystal Resonator

RSMAX

COSCIN

COSCOUT

RP

20 Ω

56pF

56pF

1-10 MΩ

25 Ω

47pF

47pF

1-10 MΩ

70 Ω

22pF

22pF

1-10 MΩ

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

5.2.2 External Clock input

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected, as

shown on Figure 15. 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).

5.2.3 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 17. Clock block diagram

Slow

Mode

%

1/2/4/8

fCPU 8/4/2/1 MHz

(or 4/2/1/0.5 MHz)

to CPU and

peripherals

x2

SMS[1:0]

%3

OSC12/6

12 or

6 MHz

Crystal

0

%2

1

MCO pin

6 MHz (USB)

19/71

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