st62t25c STMicroelectronics, st62t25c Datasheet - Page 24

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

ST62T15C/T25C/E25C
DIGITAL WATCHDOG (Cont’d)
3.3.1 Digital Watchdog Register (DWDR)
Address: 0D8h — Read/Write
Reset status: 1111 1110b
Bit 0 = C: Watchdog Control bit
If the hardware option is selected, this bit is forced
high and the user cannot change it (the Watchdog
is always active). When the software option is se-
lected, the Watchdog function is activated by set-
ting bit C to 1, and cannot then be disabled (save
by resetting the MCU).
When C is kept low the counter can be used as a
7-bit timer.
This bit is cleared to “0” on Reset.
Bit 1 = SR: Software Reset bit
This bit triggers a Reset when cleared.
When C = “0” (Watchdog disabled) it is the MSB of
the 7-bit timer.
This bit is set to “1” on Reset.
Bits 2-7 = T5-T0: Downcounter bits
It should be noted that the register bits are re-
versed and shifted with respect to the physical
counter: bit-7 (T0) is the LSB of the Watchdog
downcounter and bit-2 (T5) is the MSB.
These bits are set to “1” on Reset.
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T0
7
T1
T2
T3
T4
T5
SR
C
0
3.3.2 Application Notes
The Watchdog plays an important supporting role
in the high noise immunity of ST62xx devices, and
should be used wherever possible. Watchdog re-
lated options should be selected on the basis of a
trade-off between application security and STOP
mode availability.
When STOP mode is not required, hardware acti-
vation without EXTERNAL STOP MODE CON-
TROL should be preferred, as it provides maxi-
mum security, especially during power-on.
When STOP mode is required, hardware activa-
tion and EXTERNAL STOP MODE CONTROL
should be chosen. NMI should be high by default,
to allow STOP mode to be entered when the MCU
is idle.
The NMI pin can be connected to an I/O line (see
Figure 18) to allow its state to be controlled by soft-
ware. The I/O line can then be used to keep NMI
low while Watchdog protection is required, or to
avoid noise or key bounce. When no more
processing is required, the I/O line is released and
the device placed in STOP mode for lowest power
consumption.
When software activation is selected and the
Watchdog is not activated, the downcounter may
be used as a simple 7-bit timer (remember that the
bits are in reverse order).
The software activation option should be chosen
only when the Watchdog counter is to be used as
a timer. To ensure the Watchdog has not been un-
expectedly activated, the following instructions
should be executed within the first 27 instructions:
jrr 0, WD, #+3
ldi WD, 0FDH