ST7265X View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

ST7265X

Low speed USB 8-bit MCU family with up to 32 KB Flash/ROM

STMicroelectronics 

ST7265x

6 SUPPLY, RESET AND CLOCK MANAGEMENT

6.1 CLOCK SYSTEM

6.1.1 General Description

The MCU accepts either a 12 MHz crystal or an

external clock signal to drive the internal oscillator.

The internal clock (fCPU) is derived from the inter-

nal oscillator frequency (fOSC), which is 12 Mhz in

Stand-alone mode and 48Mhz in USB mode.

The internal clock (fCPU) is software selectable us-

ing the CP[1:0] and CPEN bits in the MISCR1 reg-

ister.

In USBVDD power supply mode, the PLL is active,

generating a 48MHz clock to the USB. In this

mode, fCPU can be configured to be up to 8 MHz.

In VDD mode the PLL and the USB clock are disa-

bled, and the maximum frequency of fCPU is 6

MHz.

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 in the frequency

range specified for fosc. The circuit shown in Fig-

ure 17 is recommended when using a crystal, and

Table 7 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

stabilisation time.

6.1.2 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

does 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

OSCIN

OSCOUT

NC

EXTERNAL

CLOCK

Figure 17. Crystal Resonator

OSCIN OSCOUT

Table 7. Recommended Values for 12-MHz

Crystal Resonator

RSMAX

COSCIN

COSCOUT

20 Ω

56pF

56pF

25 Ω

47pF

47pF

70 Ω

22pF

22pF

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

COSCIN

COSCOUT

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