ST62T35B ST Microelectronics, ST62T35B Datasheet - Page 18

• Manufacturer Part Number: ST62T35B
• Description: (ST62E35B / ST62T35B) 8-BIT OTP/EPROM MCUs
• Manufacturer: ST Microelectronics

17
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 (f
function of V
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. C
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 (f
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 f
12 as may be seen inFigure 11..
With an 8MHz oscillator frequency, the fastest
machine 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 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.
18/82
DD
, in order to guarantee correct op-
INT
divided either by 6 or by
L1
an C
L2
INT
should have
) is divided
INT
) as a
Figure 8. Oscillator Configurations
C
CRYSTAL/RESON ATOR CLOCK
L1n
INTEGR ATED CLOCK
OSG ENABLED option
OSC
OSC
EXTER NAL CLOCK
OSC
in
in
in
ST6xxx
ST6xxx
ST6xxx
OSC
OSC
OSC
NC
NC
out
out
out
C
L2
VA0015A
VA0016