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參數(shù)資料
| 型號： | ST72121J2 |
| 元件分類： | 8位微控制器 |
| 英文描述： | 8-bit Microcontroller with 2/4/8K Bytes In-System Programmable Flash |
| 中文描述： | 8位微控制器與2/4/8K字節(jié)在系統(tǒng)可編程閃存 |
| 文件頁數(shù)： | 15/93頁 |
| 文件大?。?/td> | 915K |
| 代理商： | ST72121J2 |
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15/93
ST72E121 ST72T121
3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES
3.1 CLOCK SYSTEM
3.1.1 General Description
The MCU accepts either a crystal or ceramic reso-
nator, or an external clock signal to drive the inter-
nal oscillator. The internal clock (f
CPU
) is derived
from the external oscillator frequency (f
OSC
)
.
The
external Oscillator clock is first divided by 2, and
an additional division factor of 2, 4, 8, or 16 can be
applied, in Slow Mode, to reduce the frequency of
the f
CPU
; this clock signal 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 crystal resona-
tor in the frequency range specified for f
osc
. The
circuit shown in
Figure 9
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.
3.1.2 External Clock
An external clock may be applied to the OSCIN in-
put with the OSCOUT pin not connected, as
shown on
Figure 8
.
Table 4 Recommended Values for 16 MHz
Crystal Resonator (C
0
< 7pF)
R
SMAX
40
C
OSCIN
56pF
C
OSCOUT
56pF
R
SMAX
:
Parasitic series resistance of the quartz
crystal (upper limit).
C
0
:
Parasitic shunt capacitance of the quartz crys-
tal (upper limit 7pF).
C
OSCOUT
, C
OSCIN
:
Maximum total capacitance on
pins OSCIN and OSCOUT (the value includes the
external capacitance tied to the pin plus the para-
sitic capacitance of the board and of the device).
Figure 8. External Clock Source Connections
Figure 9. Crystal/Ceramic Resonator
Figure 10. Clock Prescaler Block Diagram
60
47pF
47pF
150
22pF
22pF
OSCIN
OSCOUT
EXTERNAL
CLOCK
NC
OSCIN
OSCOUT
C
OSCIN
C
OSCOUT
OSCIN
OSCOUT
C
OSCIN
C
OSCOUT
%2
% 2, 4, 8, 16
f
to CPU and
Peripherals
15
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