L9805E Просмотр технического описания (PDF) - STMicroelectronics
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L9805E Datasheet PDF : 127 Pages
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L9805E
Clocks, Reset, Interrupts & Power saving modes
3
Clocks, Reset, Interrupts & Power saving modes
3.1
3.1.1
Note:
Clock system
General Description
The MCU accepts either a Crystal or Ceramic resonator, or an external clock signal to drive
the internal 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 (except the CAN). 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
resonator in the frequency range specified for fosc. The circuit shown in Figure 6 is
recommended when using a crystal, and Table 2 lists the recommended 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 recommended when crystals outside the specified
frequency ranges are to be used.
Table 2. Recommended Values for 16 MHz Crystal Resonator
RSMAX
COSCIN
COSCOUT
RP
40 Ω
56pF
56pF
1-10 MΩ
60 Ω
47pF
47pF
1-10 MΩ
150 Ω
22pF
22pF
1-10 MΩ
RSMAX is the equivalent serial resistor of the crystal (see crystal specification).
COSCIN,COSCOUT: Maximum total capacitances on pins OSCIN and OSCOUT (the value
includes the external capacitance tied to the pin plus the parasitic capacitance of the board
and of the device).
Rp: External shunt resistance. Recommended value for oscillator stability is 1MΩ.
Figure 5. External Clock Source Connections
OSCin
OSCout
NC
EXTERNAL
CLOCK
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