ST72F63 STMICROELECTRONICS [STMicroelectronics], ST72F63 Datasheet - Page 19

• Manufacturer Part Number: ST72F63
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

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 (f
rived from the external oscillator frequency (f
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
to
The internal clock signal (f
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 f
The circuit shown in
when using a crystal, and
ed Values for 24 MHz Crystal Resonator"
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 6. Recommended Values for 24 MHz
Crystal Resonator
Note: R
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
not apply when using an external clock input. The
equivalent specification of the external clock
C
Figure
C
R
OSCOUT
OSCIN
SMAX
R
P
SMAX
15).
Figure
is the equivalent serial resistor of the
1-10 M
56pF
56pF
20
13. The t
Figure 14
Table 6, "Recommend-
OXOV
1-10 M
CPU
47pF
47pF
25
) is also routed to
is recommended
specifications do
CPU
1-10 M
22pF
22pF
70
) is de-
lists the
OSC
osc
),
.
source should be used instead of t
tion 6.5 CONTROL TIMING).
Figure 13. External Clock Source Connections
Figure 14. Crystal/Ceramic Resonator
Figure 15. Clock Block Diagram
12 MHz
Crystal
24 or
C
OSCIN
%3
%2
OSCIN
EXTERNAL
CLOCK
OSCIN
OSC24/12
R
0
P
1
%2
OSCOUT
OSCOUT
NC
%2
%2
C
SMS
OSCOUT
1
0
OXOV
8, 4 or 2 MHz
CPU and
peripherals)
6 MHz (USB)
ST7263B
(see Sec-
19/132