st62t25c STMicroelectronics, st62t25c Datasheet - Page 15

• Manufacturer Part Number: st62t25c
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Oscillator Safeguard, Safe Reset And 28 Pins
• Manufacturer: STMicroelectronics

CLOCK SYSTEM (Cont’d)
Turning on the main oscillator is achieved by re-
setting the OSCOFF bit of the A/D Converter Con-
trol Register or by resetting the MCU. Restarting
the main oscillator 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 reason
(main oscillator defective, no clock circuitry provid-
ed, main oscillator switched off...).
User code, normal interrupts, WAIT and STOP in-
structions, are processed as normal, at the re-
duced f
cy is decreased, since the internal frequency is be-
low 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.
ADCR
Address: 0D1h — Read/Write
Bit 7-3, 1-0= ADCR7-ADCR3, ADCR1-ADCR0:
ADC Control Register . These bits are not used.
Bit 2 = OSCOFF. When low, this bit enables main
oscillator to run. The main oscillator is switched off
when OSCOFF is high.
3.1.3 Oscillator Safe Guard
The Oscillator Safe Guard (OSG) affords drastical-
ly increased operational integrity in ST62xx devic-
es. The OSG circuit provides three basic func-
ADCR
7
7
ADCR
(LFAO)
LFAO
6
frequency. The A/D converter accura-
ADCR
5
ADCR
4
ADCR
LFAO
3
OSC
clock frequency.
OFF
ADCR
1
ADCR
0
0
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 fre-
quency low cost oscillator; finally, it automatically
limits the internal clock frequency as a function of
supply voltage, in order to ensure correct opera-
tion 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 effectively
increased internal clock frequency. In the absence
of an OSG circuit, this may lead to an over fre-
quency for a given power supply voltage. The
OSG filters out such spikes (as illustrated in Figure
9). In all cases, when the OSG is active, the maxi-
mum internal clock frequency, f
f
lationship is illustrated in Figure 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
cy with OSG enabled.
Note. The OSG should be used wherever possible
as it provides maximum safety. Care must be tak-
en, however, as it can increase power consump-
tion and reduce the maximum operating frequency
to f
Warning: Care has to be taken when using the
OSG, as the internal frequency is defined between
a minimum and a maximum value and is not accu-
rate.
For precise timing measurements, it is not recom-
mended to use the OSG and it should not be ena-
bled in applications that use the SPI or the UART.
It should also be noted that power consumption in
Stop mode is higher when the OSG is enabled
(around 50 A at nominal conditions and room
temperature).
OSG
OSG
, which is supply voltage dependent. This re-
.
OSG
: the maximum authorised frequen-
ST62T15C/T25C/E25C
INT
, is limited to
15/70
DD
15
),