ST72P623 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

ST72P623

LOW SPEED USB 8-BIT MCU WITH 3 ENDPOINTS, FLASH OR ROM MEMORY, LVD, WDG, 10-BIT ADC, 2 TIMERS, SCI, SPI

STMicroelectronics 

ST7262xxx

6 CLOCKS AND RESET

6.1 CLOCK SYSTEM

6.1.1 General Description

6.1.2 External Clock input

The MCU accepts either a Crystal or Ceramic res- An external clock may be applied to the OSCIN in-

onator, or an external clock signal to drive the in- put with the OSCOUT pin not connected, as

ternal oscillator. The internal clock (fCPU) is de- shown on Figure 12. The tOXOV specifications

rived from the external oscillator frequency (fOSC),

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

does not apply when using an external clock input.

The equivalent specification of the external clock

OSC12/6 bit in the option byte, a 12 MHz external source should be used instead of tOXOV (see Elec-

clock can be used giving an internal frequency of 8

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

) fer to Figure 14).

t(s The internal clock signal (fCPU) consists of a

c square wave with a duty cycle of 50%.

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

ro Slow Mode Selection bits in the Miscellaneous

register (SMS[1:0])

P The internal oscillator is designed to operate with

te an AT-cut parallel resonant quartz or ceramic res-

le onator in the frequency range specified for fosc.

o The circuit shown in Figure 13 is recommended

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

b mended capacitors. The crystal and associated

O components should be mounted as close as pos-

- sible to the input pins in order to minimize output

) distortion and start-up stabilization time.

ct(s Table 4. Recommended Values for 12 MHz

u Crystal Resonator

rod RSMAX

P COSCIN

te COSCOUT

le RP

20 Ω

56pF

56pF

1-10 MΩ

25 Ω

47pF

47pF

1-10 MΩ

70 Ω

22pF

22pF

1-10 MΩ

so Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

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

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

OSCIN

OSCOUT

NC

EXTERNAL

CLOCK

Figure 13. Crystal/Ceramic Resonator

OSCIN OSCOUT

stress the crystal, ST recommends to add a serial

resistor on the OSCOUT pin to limit the drive level

COSCIN

COSCOUT

in accordance with the crystal manufacturer’s

specification. Please also refer to Section 12.5.4.

20/139

Doc ID 6996 Rev 5

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