(5) Do not apply 12 V to the V
PP
pin during normal operation. To prevent microcontroller errors
caused by accidental programming or erasing, apply 12 V to V
PP
only when the flash memory is
programmed or erased, or when flash memory is emulated by RAM. While 12 V is applied, the
watchdog timer should be running and enabled to halt runaway program execution, so that
program runaway will not lead to overprogramming or overerasing.
(6) Disable watchdog-timer reset output (
RESO
) while the programming voltage (V
PP
) is turned
on. If 12 V is applied during watchdog timer reset output (while the
RESO
pin is low), overcurrent
flow will permanently destroy the reset output circuit. The watchdog timers reset output enable bit
(RSTOE) should not be set to 1.
If a pull-up resistor is externally attached to the V
PP
/
RESO
pin, a diode is necessary to prevent
reverse current from flowing to V
CC
when V
PP
is applied (figure 18-31).
(7) If the watchdog timer generates a reset output signal when 12 V is not applied, the rise and
fall of the reset output waveform will be delayed by any decoupling capacitors connected to the
V
PP
pin.
Figure 18-31 V
PP
Power Supply Circuit Design (Example)
0.01
µ
F
1.0
µ
F
+12 V
V /
RESO
PP
H8/3048F
+5 V
Pull-up resistor and
a diode
628
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