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ST7263B
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6.2 CLOCK SYSTEM
6.2.1 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). The internal
clock is further divided by 2 by setting the SMS bit
in the Miscellaneous Register.
Using the OSC24/12 bit in the option byte, a 12
MHz or a 24 MHz external clock can be used to
provide an internal frequency of either 2, 4 or 8
MHz while maintaining a 6 MHz for the USB (refer
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 16 is recommended
recommended capacitance. The crystal and asso-
ciated components should be mounted as close as
possible to the input pins in order to minimize out-
put distortion and start-up stabilisation time.
Table 7. Recommended Values for 24 MHz
Crystal Resonator
Note: RSMAX is the equivalent serial resistor of the
crystal (see crystal specification).
6.2.2 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 do 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/Ceramic Resonator
Figure 17. Clock Block Diagram
RSMAX
20
25
70
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
%3
CPU and
8, 4 or 2 MHz
6 MHz (USB)
24 or
peripherals)
%2
1
0
%2
12 MHz
Crystal
%2
0
1
OSC24/12
SMS
%2