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ST62T28C/E28C
3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES
3.1 CLOCK SYSTEM
The MCU featuresa 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
NET
). In addition, a Low Frequency Auxiliary Os-
cillator (LFAO) can be switched in for security rea-
sons, to reduce power consumption, orto offerthe
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
INT
) as a
function ofV
DD
, inorder toguarantee correct oper-
ation. These functions are illustrated in Figure 2,
Figure 3, Figure 4 and Figure 5.
Figure 1 illustrates various possible oscillator con-
figurations using anexternal crystal or ceramic res-
onator, an external clock input, anexternal resistor
(R
NET
), or the lowest cost solution using only the
LFAO. C
L1
anC
L2
shouldhave acapacitance inthe
range 12 tST6_CLK1o 22 pF for an oscillator fre-
quency in the 4-8 MHz range.
The internal MCU clock frequency (f
INT
) is divided
by 12to drive the Timer, theA/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, thefastest ma-
chine cycle is therefore 1.625
μ
s.
A machine cycle is thesmallest unitof timeneeded
to executeanyoperation(forinstance,to increment
the Program Counter). An instruction may require
two, four, or five machine cycles for execution.
3.1.1 Main Oscillator
The oscillatorconfigurationmaybe specifiedbyse-
lectingtheappropriate option.WhentheCRYSTAL/
RESONATORoptionisselected,itmustbeusedwith
a quartz crystal, a ceramicresonator oran external
signalprovidedontheOSCinpin.When theRCNET-
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.
Figure 8. Oscillator Configurations
INTEGRATED CLOCK
CRYSTAL/RESONATOR option
OSG ENABLED option
OSC
in
OSC
out
C
L1n
C
L2
ST6xxx
CRYSTAL/RESONATOR CLOCK
CRYSTAL/RESONATOR option
OSC
in
OSC
out
ST6xxx
EXTERNAL CLOCK
CRYSTAL/RESONATOR option
NC
OSC
in
OSC
out
ST6xxx
NC
OSC
in
OSC
out
R
NET
ST6xxx
RC NETWORK
RC NETWORK option
NC