ST62T35B ST Microelectronics, ST62T35B Datasheet - Page 19

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

CLOCK SYSTEM (Cont’d)
Turning on the main oscillator is achieved by re-
setting the OSCOFF bit of the OSCR Register or
by resetting the MCU. Restarting the main oscilla-
tor implies a delay comprising the oscillator start
up delay period plus the duration of the software
instruction at f
3.1.2 Low Frequency Auxiliary Oscillator
The Low Frequency Auxiliary Oscillator has three
main purposes. Firstly, it can be used to reduce
power consumption in non timing critical routines.
Secondly, it offers a fully integrated system clock,
without any external components. Lastly, it acts as
a safety oscillator in case of main oscillator failure.
This oscillator is available when the OSG ENA-
BLED option is selected. In this case, it automati-
cally starts one of its periods after the first missing
edge from the main oscillator, whatever the rea-
son (main oscillator defective, no clock circuitry
provided, main oscillator switched off...).
User code, normal interrupts, WAIT and STOP in-
structions, are processed as normal, at the re-
duced f
racy is decreased, since the internal frequency is
below 1MHz.
At power on, the Low Frequency Auxiliary Oscilla-
tor starts faster than the Main Oscillator. It there-
fore feeds the on-chip counter generating the
POR delay until the Main Oscillator runs.
The Low Frequency Auxiliary Oscillator is auto-
matically switched off as soon as the main oscilla-
tor starts.
OSCR
Address: 0DBh — Read/Write
Bit 7-1= These bits are not used and must be kept
cleared after reset.
Bit 0 = OSCOFF . Main oscillator turn-off. When
low, this bit enables main oscillator to run. The
main oscillator is switched off when OSCOFF is
high.
7
-
(LFAO)
LFAO
-
frequency. The A/D converter accu-
LFAO
-
clock frequency.
-
-
-
-
OSC
OFF
0
3.1.3 Oscillator Safe Guard
The Oscillator Safe Guard (OSG) affords drasti-
cally increased operational integrity in ST62xx de-
vices. The OSG circuit provides three basic func-
tions: it filters spikes from the oscillator lines which
would result in over frequency to the ST62 CPU; it
gives access to the Low Frequency Auxiliary Os-
cillator (LFAO), used to ensure minimum process-
ing in case of main oscillator failure, to offer re-
duced power consumption or to provide a fixed
frequency low cost oscillator; finally, it automati-
cally limits the internal clock frequency as a func-
tion of supply voltage, in order to ensure correct
operation even if the power supply should drop.
The OSG is enabled or disabled by choosing the
relevant OSG option. It may be viewed as a filter
whose cross-over frequency is device dependent.
Spikes on the oscillator lines result in an effective-
ly increased internal clock frequency. In the ab-
sence of an OSG circuit, this may lead to an over
frequency for a given power supply voltage. The
OSG filters out such spikes (as illustrated inFig-
ure 9.). In all cases, when the OSG is active, the
maximum internal clock frequency, f
to f
relationship is illustrated inFigure 12..
When the OSG is enabled, the Low Frequency
Auxiliary Oscillator may be accessed. This oscilla-
tor starts operating after the first missing edge of
the main oscillator (see Figure 10.).
Over-frequency, at a given power supply level, is
seen by the OSG as spikes; it therefore filters out
some cycles in order that the internal clock fre-
quency of the device is kept within the range the
particular device can stand (depending on V
and below f
quency with OSG enabled.
Note. The OSG should be used wherever possi-
ble as it provides maximum safety. Care must be
taken, however, as it can increase power con-
sumption and reduce the maximum operating fre-
quency to f
OSG
, which is supply voltage dependent. This
OSG
OSG
.
: the maximum authorised fre-
ST62T35B/E35B
INT
, is limited
19/82
DD
18
),