ST7261 datasheet(13/80 Pages) STMICROELECTRONICS

13 / 80 page

ST7261

13/80

5 CLOCKS AND RESET

5.1 CLOCK SYSTEM

5.1.1 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 10).

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 9 is recommended

when using a crystal, and Table 3 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 3. Recommended Values for 12 MHz

Crystal Resonator

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

5.1.2 External Clock input

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected, as

shown on Figure 8. 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).

5.1.3 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 8. External Clock Source Connections

Figure 9. Crystal/Ceramic Resonator

Figure 10. Clock block diagram

RSMAX

Ω

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

1-10 M

Ω

1-10 M

Ω

1-10 M

Ω

OSCIN

OSCOUT

EXTERNAL

CLOCK

OSCIN

OSCOUT

COSCIN

COSCOUT

to CPU and

fCPU 8/4/2/1 MHz

6 MHz (USB)

12 or

peripherals

OSC12/6

6 MHz

Crystal

Slow

Mode

SMS[1:0]

1/2/4/8

(or 4/2/1/0.5 MHz)

MCO pin

ST7261 数据表(PDF) 13 Page - STMicroelectronics

ST7261 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

数据表下载

链接网址

ST7261 数据表(PDF) 13 Page - STMicroelectronics

ST7261 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

数据表 下载

链接网址

数据表下载