HD64F3437TF16 Datasheet, PDF (521/752 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.

 The P and E bits must be cleared in the flash memory control register (FLMCR)
When applying or releasing V_PP, make sure that the P or E bit is not set by mistake.
 No program runaway
When V_PP is applied, program execution must be supervised, e.g. by the watchdog timer.
These power-on and power-off timing requirements should also be satisfied in the event of a
power failure and in recovery from a power failure. If these requirements are not satisfied,
overprogramming or overerasing may occur due to program runaway etc., which could cause
memory cells to malfunction.
• The V_PP flag is set and cleared by a threshold decision on the voltage applied to the FV_PP pin.
The threshold level is between approximately V_CC + 2 V to 11.4 V.
When this flag is set, it becomes possible to write to the flash memory control register
(FLMCR) and the erase block registers (EBR1 and EBR2), even though the V_PP voltage may
not yet have reached the programming voltage range of 12.0 ± 0.6 V.
Do not actually program or erase the flash memory until V_PP has reached the programming
voltage range.
The programming voltage range for programming and erasing flash memory is 12.0 ± 0.6 V
(11.4 V to 12.6 V). Programming and erasing cannot be performed correctly outside this range.
When not programming or erasing the flash memory, ensure that the V_PP voltage does not
exceed the V_CC voltage. This will prevent unintended programming and erasing.
In this chip, the same pin is used for STBY and FV_PP. When this pin is driven low, a transition
is made to hardware standby mode. This happens not only in the normal operating modes
(modes 1, 2, and 3), but also when programming the flash memory with a PROM programmer.
When programming with a PROM programmer, therefore, use a programmer which sets this
pin to the V_CC level when not programming (FV_PP=12 V).
Note:
Here, V_PP application, release, and cutoff are defined as follows:
Application: Raising the voltage from V_CC to 12±0.6 V.
Release: Dropping the voltage from 12±0.6 V to V_CC.
Cutoff:
Halting voltage application (setting the floating state).
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