参数资料
型号: ST72651AR6T1E/XXX
厂商: STMICROELECTRONICS
元件分类: 微控制器/微处理器
英文描述: 8-BIT, MROM, MICROCONTROLLER, PQFP64
封装: 10 X 10 MM, ROHS COMPLIANT, TQFP-64
文件页数: 81/161页
文件大小: 2656K
代理商: ST72651AR6T1E/XXX
ST72651AR6
26/161
Doc ID 7215 Rev 4
6 SUPPLY, RESET AND CLOCK MANAGEMENT
6.1 CLOCK SYSTEM
6.1.1 General Description
The MCU accepts either a 12 MHz crystal or an
external clock signal to drive the internal oscillator.
The internal clock (fCPU) is derived from the inter-
nal oscillator frequency (fOSC), which is 12 Mhz in
Stand-alone mode and 48Mhz in USB mode.
The internal clock (fCPU) is software selectable us-
ing the CP[1:0] and CPEN bits in the MISCR1 reg-
ister.
In USBVDD power supply mode, the PLL is active,
generating a 48MHz clock to the USB. In this
mode, fCPU can be configured to be up to 8 MHz.
In VDD mode the PLL and the USB clock are disa-
bled, and the maximum frequency of fCPU is 6
MHz.
The internal clock signal (fCPU) 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 in the frequency
range specified for fosc. The circuit shown in Fig-
ure 16 is recommended when using a crystal, and
Table 7 lists the recommended capacitance. 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.
Table 7. Recommended Values for 12-MHz
Crystal Resonator
Note: RSMAX is the equivalent serial resistor of the
crystal (see crystal specification).
6.1.2 External Clock
An external clock may be applied to the OSCIN in-
put with the OSCOUT pin not connected, as
shown on Figure 15. 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 Sec-
tion 6.5 CONTROL TIMING).
Figure 15. External Clock Source Connections
Figure 16. Crystal Resonator
RSMAX
20
Ω
25
Ω
70
Ω
COSCIN
56pF
47pF
22pF
COSCOUT
56pF
47pF
22pF
OSCIN
OSCOUT
EXTERNAL
CLOCK
NC
OSCIN
OSCOUT
COSCIN
COSCOUT
1
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