HD64F3437TF16 Datasheet, PDF (580/752 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.

22.4 Hardware Standby Mode
22.4.1 Transition to Hardware Standby Mode
Regardless of its current state, the chip enters hardware standby mode whenever the STBY pin
goes low.
Hardware standby mode reduces power consumption drastically by halting the CPU, stopping all
the functions of the on-chip supporting modules, and placing I/O ports in the high-impedance
state. The registers of the on-chip supporting modules are reset to their initial values. Only the on-
chip RAM is held unchanged, provided the minimum necessary voltage supply is maintained.
Notes: 1. The RAME bit in the system control register should be cleared to 0 before the STBY
pin goes low.
2. Do not change the inputs at the mode pins (MD₁, MD₀) during hardware standby mode.
Be particularly careful not to let both mode pins go low in hardware standby mode,
since that places the chip in writer mode and increases current dissipation.
22.4.2 Recovery from Hardware Standby Mode
Recovery from the hardware standby mode requires inputs at both the STBY and RES pins. When
the STBY pin goes high, the clock oscillator begins running. The RES pin should be low at this
time and should be held low long enough for the clock to stabilize. When the RES pin changes
from low to high, the reset sequence is executed and the chip returns to the program execution
state.
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