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参数资料
| 型号： | ST72671N4B1/XXX |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, MROM, 8 MHz, MICROCONTROLLER, PDIP56 |
| 封装： | 0.600 INCH, SHRINK, PLASTIC, DIP-56 |
| 文件页数： | 8/101页 |
| 文件大小： | 608K |
| 代理商： | ST72671N4B1/XXX |
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ST72671
3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES
3.1 CLOCK SYSTEM
3.1.1 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).
The external Oscillator clock is first divided by 3,
and an additional division factor of 2 can be ap-
plied if Slow Mode is selected by setting the SMS
bit in the Miscellaneous Register. This reduces the
frequency of the fCPU; the clock signal is also rout-
ed to the on-chip peripherals. The CPU clock sig-
nal 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 crystal resona-
tor in the frequency range specified for fosc.The
circuit shown in Figure 7 is recommended when
using a crystal, and Table 4 lists the recommend-
ed capacitance and feedback resistance values.
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.
Use of an external CMOS oscillator is recom-
mended when crystals outside the specified fre-
quency ranges are to be used.
Table 4
. Recommended Crystal Values
Legend:
CL1,CL2 = Maximum total capacitance on pins
OSCIN and OSCOUT (the value includes the ex-
ternal capacitance tied to the pin plus the parasitic
capacitance of the board and of the device).
RSMAX = Maximum series parasitic resistance of
the quartz allowed.
Note: The tables relate to the quartz crystal only
(not ceramic resonator).
3.1.2 External Clock
An external clock may be applied to the OSCIN in-
put with the OSCOUT pin not connected. The tOX-
OV specifications does not apply when using an
external clock input. The equivalent specification
of the external clock source should be used in-
stead of tOXOV (see Electrical Characteristics).
Figure 7. Crystal/Ceramic Resonator
Figure 8. Clock Prescaler Block Diagram
24 Mhz
Unit
RSMAX
70
25
20
Ohms
CL1
22
47
56
pf
CL2
22
47
56
pf
OSCIN
OSCOUT
COSCIN
COSCOUT
RP
OSCIN
OSCOUT
COSCIN
COSCOUT
RP
%3
% 2
CPUCLK
to CPU and
Peripherals
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