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21.5
Hardware Standby Mode
21.5.1
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 all functions
of the CPU, and on-chip supporting modules. All modules are reset except the on-chip RAM. As
long as the specified voltage is supplied, on-chip RAM data is retained. I/O ports are placed in the
high-impedance state.
Clear the RAME bit to 0 in SYSCR before STBY goes low to retain on-chip RAM data.
The inputs at the mode pins (MD2 to MD0) should not be changed during hardware standby
mode.
21.5.2
Exit from Hardware Standby Mode
Hardware standby mode is exited by inputs at the STBY and RES pins. While RES is low, when
STBY goes high, the clock oscillator starts running. RES should be held low long enough for the
clock oscillator to settle. When RES goes high, reset exception handling begins, followed by a
transition to the program execution state.
21.5.3
Timing for Hardware Standby Mode
Figure 21.2 shows the timing relationships for hardware standby mode. To enter hardware standby
mode, first drive RES low, then drive STBY low. To exit hardware standby mode, first drive
STBY high, wait for the clock to settle, then bring RES from low to high.
Clock
oscillator
RES
STBY
Figure 21.2 Hardware Standby Mode Timing
Oscillator
settling time
Reset
exception
handling
639
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