505
17.5
Hardware Standby Mode
17.5.1
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 (MD
2
to MD
0
) should not be changed during hardware standby mode.
17.5.2
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.
17.5.3
Timing for Hardware Standby Mode
Figure 17-2 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.
RES
STBY
Clock
oscillator
Oscillator
settling time
Reset
exception
handling
Figure 17-2 Hardware Standby Mode Timing