HD64F3437TF16 Datasheet, PDF (520/752 Pages) Hitachi Semiconductor – 12 V must not be applied to the S-mask model (single-power-supply specification), as this may permanently damage the device.

(4) Set H'FF as the PROM programmer buffer data for addresses H'F780 to H'1FFFF. The
H8/3437F PROM size is 60 kbytes. Addresses H'F780 to H'1FFFF always read H'FF, so if H'FF is
not specified as programmer data, a verify error will occur.
(5) Notes on applying, releasing, and shutting off the programming voltage (V_PP)
Note:
In this section, the application, release, and shutting-off of V_PP are defined as follows.
Application: A rise in voltage from V_CC to 12 V ± 0.6 V.
Release: A drop in voltage from 12 V ± 0.6 V to V_CC.
Shut-off: No applied voltage (floating).
• Apply the programming voltage (V_PP) after the rise of V_CC, and release V_PP before shutting off
V_CC.
To prevent unintended programming or erasing of flash memory, in these power-on and
power-off timings, the application, release, and shutting-off of V_PP must take place when the
microcontroller is in a stable operating condition as defined below.
Stable operating condition
 The V_CC voltage must be stabilized within the rated voltage range (V_CC = 2.7 V to 5.5 V)*
If V_PP is applied, released, or shut off while the microcontroller’s V_CC voltage is not within
the rated voltage range (V_CC = 2.7 to 5.5 V)*, since microcontroller operation is unstable,
the flash memory may be programmed or erased by mistake. This can occur even if V_CC =
0 V. To prevent changes in the V_CC power supply when V_PP is applied, be sure that the
power supply is adequately decoupled with inserting bypass capacitors.
Note: * In the LH version, V_CC = 3.0 V to 5.5 V.
 Clock oscillation must be stabilized (the oscillation settling time must have elapsed), and
oscillation must not be stopped
When turning on V_CC power, hold the RES pin low during the oscillation settling time
(t_OSC1 = 20 ms), and do not apply V_PP until after this time.
 The microcontroller must be in the reset state, or in a state in which a reset has ended
normally (reset has been released) and flash memory is not being accessed
Apply or release V_PP either in the reset state, or when the CPU is not accessing flash
memory (when a program in on-chip RAM or external memory is executing). Flash
memory cannot be read normally at the instant when V_PP is applied or released. Do not read
flash memory while V_PP is being applied or released.
For a reset during operation, apply or release V_PP only after the RES pin has been held low
for at least ten system clock cycles (10ø).
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