ST72T311 ST Microelectronics, ST72T311 Datasheet - Page 17

• Manufacturer Part Number: ST72T311
• Description: 8-BIT MCU WITH 8 TO 16K OTP/EPROM
• Manufacturer: ST Microelectronics

3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES
3.1 CLOCK SYSTEM
3.1.1 General Description
The MCU accepts either a crystal or ceramic reso-
nator, or an external clock signal to drive the inter-
nal oscillator. The internal clock (f
from the external oscillator frequency (f
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 f
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 crystal resona-
tor in the frequency range specified for f
circuit shown in Figure 11 is recommended when
using a crystal, and Table 5 lists the recommend-
ed 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 recom-
mended when crystals outside the specified fre-
quency ranges are to be used.
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 10.
Table 5 Recommended Values for 16 MHz
R
crystal (upper limit).
C
tal (upper limit 7pF).
C
pins OSCIN and OSCOUT (the value includes the
external capacitance tied to the pin plus the para-
sitic capacitance of the board and of the device).
SMAX
0
OSCOUT
C
: Parasitic shunt capacitance of the quartz crys-
C
R
OSCOUT
CPU
OSCIN
SMAX
: Parasitic series resistance of the quartz
; this clock signal is also routed to the on-
, C
Crystal Resonator (C
OSCIN
56pF
56pF
40
: Maximum total capacitance on
47pF
47pF
60
0
< 7pF)
CPU
) is derived
OSC
150
22pF
22pF
osc
)
. The
.
The
Figure 10. External Clock Source Connections
Figure 11. Crystal/Ceramic Resonator
Figure 12. Clock Prescaler Block Diagram
C
OSCIN
OSCIN
C
OSCIN
OSCOUT
EXTERNAL
OSCIN
CLOCK
OSCIN
C
OSCOUT
%2
ST72E311 ST72T311
OSCOUT
OSCOUT
NC
% 2, 4, 8, 16
C
OSCOUT
f
to CPU and
Peripherals
CPU
17
17/100