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ST62P35BQ1 View Datasheet(PDF) - STMicroelectronics

Part Name
Description
MFG CO.
'ST62P35BQ1' PDF : 82 Pages View PDF
ST62T35B/E35B
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. In addition, a Low Frequen-
cy Auxiliary Oscillator (LFAO) can be switched in
for security reasons, to reduce power consump-
tion, or to offer the benefits of a back-up clock sys-
tem.
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 (IfNT) as a
function of VDD, in order to guarantee correct op-
eration. These functions are illustrated in Figure
9., Figure 10., Figure 11. and Figure 12..
Figure 8. illustrates various possible oscillator con-
figurations using an external crystal or ceramic res-
onator, an external clock input or the lowest cost so-
lution using only the LFAO. CL1 an CL2 should have
a capacitance in the range 12 to 22 pF for an os-
cillator frequency in the 4-8 MHz range.
The internal MCU clock frequency (fINT) is divided
by 12 to drive the Timer and the Watchdog timer,
and by 13 to drive the CPU core, while the A/D
converter is driven by fINT divided either by 6 or by
12 as may be seen inFigure 11..
With an 8MHz oscillator frequency, the fastest
machine cycle is therefore 1.625µs.
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 main oscillator can be turned off (when the
OSG ENABLED option is selected) by setting the
OSCOFF bit of the OSCR Control Register. The
Low Frequency Auxiliary Oscillator is automatical-
ly started.
Figure 8. Oscillator Configurations
CRYSTAL/RESON ATOR CLOCK
ST6xxx
OSCin
OSCout
CL1n
CL2
VA0016
EXTER NAL CLOCK
ST6xxx
OSCin
OSCout
NC
VA 0015A
INTEGR ATED CLOCK
OSG ENABLED option
ST6xxx
OSCin
OSCout
NC
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