C541U_99 Datasheet, PDF (168/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

Power Saving Modes
C541U
9.2 Power Down Mode
In the power down mode, the RC osciillator and the on-chip oscillator which operates with the XTAL
pins is stopped. Therefore, all functions of the microcontroller are stopped and only the contents of
the on-chip RAM, XRAM and the SFR’s are maintained. The port pins, which are controlled by their
port latches, output the values that are held by their SFR’s. The port pins which serve the alternate
output functions show the values they had at the end of the last cycle of the instruction which
initiated the power down mode. ALE and PSEN hold at logic low level (see table 9-1). The power
down mode can be left either by an active reset signal or by a low signal at the P3.2/INT0 pin or any
activity on the USB bus. The USB module enters the suspend state when it detects no activity on the
USB bus for more than 6 ms. The suspend state is left when bus activity is detected on the USB bus.
Leaving the suspend state can (if selected and enabled) wake-up the power down mode.
Using reset to leave power down mode puts the microcontroller with its SFRs into the reset state.
Using the INT0 pin or USB bus for power down mode exit maintains the state of the SFRs, which
has been frozen when power down mode is entered.
In the power down mode of operation, V_DD can be reduced to minimize power consumption. It must
be ensured, however, that is V_DD not reduced before the power down mode is invoked, and that V_DD
is restored to its normal operating level before the power down mode is terminated.
Semiconductor Group
9-5
1997-10-01