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| 型号： | ST72621K4B1 |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PDIP32 |
| 封装： | 0.400 INCH, LEAD FREE, SHRINK, PLASTIC, DIP-32 |
| 文件页数： | 53/139页 |
| 文件大小： | 1264K |
| 代理商： | ST72621K4B1 |
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ST7262xxx
20/139
Doc ID 6996 Rev 5
6 CLOCKS AND RESET
6.1 CLOCK SYSTEM
6.1.1 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 14).
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
register (SMS[1:0])
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 13 is recommended
when using a crystal, and Table 4 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 4. Recommended Values for 12 MHz
Crystal Resonator
Note: RSMAX is the equivalent serial resistor of the
crystal (see crystal specification).
Note: When a crystal is used, and to not over-
stress the crystal, ST recommends to add a serial
resistor on the OSCOUT pin to limit the drive level
in accordance with the crystal manufacturer’s
specification. Please also refer to Section 12.5.4.
6.1.2 External Clock input
An external clock may be applied to the OSCIN in-
put with the OSCOUT pin not connected, as
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).
6.1.3 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 12. External Clock Source Connections
Figure 13. Crystal/Ceramic Resonator
RSMAX
20
Ω
25
Ω
70
Ω
COSCIN
56pF
47pF
22pF
COSCOUT
56pF
47pF
22pF
RP
1-10 M
Ω
1-10 M
Ω
1-10 M
Ω
OSCIN
OSCOUT
EXTERNAL
CLOCK
NC
OSCIN
OSCOUT
COSCIN
COSCOUT
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