st62t18c STMicroelectronics, st62t18c Datasheet - Page 16

• Manufacturer Part Number: st62t18c
• Description: 8-bit Mcus With A/d Converter, Auto-reload Timer, Uart, Osg, Safe Reset And 20-pin Package
• Manufacturer: STMicroelectronics

ST62T18C/E18C
3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES
3.1 CLOCK SYSTEM
The MCU features a Main Oscillator which can be
driven by an external clock, or used in conjunction
with an AT-cut parallel resonant crystal or a suita-
ble ceramic resonator, or with an external resistor
(R
cillator (LFAO) can be switched in for security rea-
sons, to reduce power consumption, or to offer the
benefits of a back-up clock system.
The Oscillator Safeguard (OSG) option filters
spikes from the oscillator lines, provides access to
the LFAO to provide a backup oscillator in the
event of main oscillator failure and also automati-
cally limits the internal clock frequency (f
function of V
ation. These functions are illustrated in Figure 2,
Figure 3, Figure 4 and Figure 5.
Figure 1 illustrates various possible oscillator con-
figurations using an external crystal or ceramic res-
onator, an external clock input, an external resistor
(R
LFAO. C
range 12 tST6_CLK1o 22 pF for an oscillator fre-
quency in the 4-8 MHz range.
The internal MCU clock frequency (f
by 12 to drive the Timer, the A/D converter and the
Watchdog timer, and by 13 to drive the CPU core,
as may be seen in Figure 4.
With an 8MHz oscillator frequency, the fastest ma-
chine cycle is therefore 1.625 s.
A machine cycle is the smallest unit of time needed
to execute any operation (for instance, to increment
the Program Counter). An instruction may require
two, four, or five machine cycles for execution.
3.1.1 Main Oscillator
The oscillator configuration may be specified by se-
lecting theappropriate option.When the CRYSTAL/
RESONATORoption isselected, itmustbeusedwith
a quartz crystal, a ceramic resonator or an external
signal providedonthe OSCinpin.When theRC NET-
WORK option is selected, the system clock is gen-
erated by an external resistor.
The main oscillator can be turned off (when the
OSG ENABLED option is selected) by setting the
OSCOFF bit of the ADC Control Register. The
Low Frequency Auxiliary Oscillator is automatical-
ly started.
16/82
16
NET
NET
). In addition, a Low Frequency Auxiliary Os-
), or the lowest cost solution using only the
L1
an C
DD
, in order to guarantee correct oper-
L2
should have a capacitance in the
INT
) is divided
INT
) as a
Figure 8. Oscillator Configurations
C
L1n
CRYSTAL/RES ONATOR CLOCK
CRYSTAL/RESON ATOR option
CRYSTAL/RESON ATOR option
CRYSTAL/RESON ATOR option
OSG ENABLED option
INTEGRA TED CLOCK
RC NETW ORK option
OSC
EXTERNAL CLOCK
OSC
OSC
OSC
NC
RC NETW ORK
in
in
in
ST6xxx
ST6xxx
ST6xxx
in
ST6xxx
OSC
OSC
OSC
OSC
NC
NC
out
out
out
out
R
C
NET
L2