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参数资料
| 型号： | ST72121J2T6/XXX |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, MROM, 16 MHz, MICROCONTROLLER, PQFP44 |
| 封装： | PLASTIC, TQFP-44 |
| 文件页数： | 6/87页 |
| 文件大小： | 1365K |
| 代理商： | ST72121J2T6/XXX |
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ST72121
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 (fCPU) is derived
from the external oscillator frequency (fOSC). 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 fCPU; 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 fosc.The
circuit shown in Figure 8 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 7.
Table 4 Recommended Values for 16 MHz
Crystal Resonator (C0 < 7pF)
RSMAX: Parasitic series resistance of the quartz
crystal (upper limit).
C0: Parasitic shunt capacitance of the quartz crys-
tal (upper limit 7pF).
COSCOUT,COSCIN: 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).
Rp: External shunt resistance. Recommended val-
ue for oscillator stability is 1M
.
Figure 7. External Clock Source Connections
Figure 8. Crystal/Ceramic Resonator
Figure 9. Clock Prescaler Block Diagram
RSMAX
40
60
150
COSCIN
56pF
47pF
22pF
COSCOUT
56pF
47pF
22pF
RP
1-10 M
1-10 M
1-10 M
OSCIN
OSCOUT
EXTERNAL
CLOCK
NC
OSCIN
OSCOUT
COSCIN
COSCOUT
RP
OSCIN
OSCOUT
COSCIN
COSCOUT
RP
%2
% 2,4, 8,16
fCPU
to CPU and
Peripherals
14
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