ST72631K4D1 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
Manufacturer
ST72631K4D1
STMicroelectronics
ST72631K4D1 Datasheet PDF : 109 Pages
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ST7263
3 CLOCKS AND RESET
3.1 CLOCK SYSTEM
3.1.1 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),
which is divided by 3 (and by 2 or 4 for USB, de-
pending on the external clock used).
By setting the CLKDIV bit in the Miscellaneous
Register, a 12 MHz external clock can be used giv-
ing an internal frequency of 4 MHz while maintain-
ing a 6 MHz for USB (refer to Figure 10).
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 or ceramic res-
onator in the frequency range specified for fosc.
The circuit shown in Figure 9 is recommended
when using a crystal, and Table 5 Recommended
Values for 24 MHz Crystal Resonator lists the rec-
ommended capacitance. The crystal and associat-
ed components should be mounted as close as
possible to the input pins in order to minimize out-
put distortion and start-up stabilisation time.
Figure 8. External Clock Source Connections
OSCIN
OSCOUT
NC
EXTERNAL
CLOCK
Figure 9. Crystal/Ceramic Resonator
OSCIN OSCOUT
RP
COSCIN
COSCOUT
Table 5. Recommended Values for 24 MHz
Crystal Resonator
RSMAX
COSCIN
COSCOUT
RP
20 Ω
56pF
56pF
1-10 MΩ
25 Ω
47pF
47pF
1-10 MΩ
70 Ω
22pF
22pF
1-10 MΩ
Note: RSMAX is the equivalent serial resistor of the
crystal (see crystal specification).
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. 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 10. Clock block diagram
8 or 4 MHz
CPU and
peripherals)
%3
24 or
12 MHz
Crystal
CLKDIV
1
%2
0
6 MHz (USB)
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