C509-L_97 Datasheet, PDF (226/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Power Saving Modes
C509-L
9 Power Saving Modes
The C509-L provides three modes in which power consumption can be significantly reduced.
– Idle mode
The CPU is gated off from the oscillator. All peripherals are still provided with the clock and
are able to work.
– Power down mode
The operation of the C509-L is completely stopped and the oscillator is turned off. This mode
is used to save the contents of the internal RAM with a very low standby current. Power down
mode can be entered by software or by hardware.
– Slow-down mode
The controller keeps up the full operating functionality, but its normal clock frequency is
internally divided by eight. This slows down all parts of the controller, the CPU and all
peripherals, to 1/8 th of their normal operating frequency. Slowing down the frequency greatly
reduces power consumption.
All of these modes - a detailed description of each is given in the following sections - are entered by
software. Special function register PCON (power control register) is used to select one of these
modes.
These power saving modes, especially the power down mode, replace the hardware power down
supply for the internal RAM via a dedicated pin. During the power saving modes, the power supply
for the C509-L are all V_CC pins. There is no further dedicated pin for power down supply.
For the C509-L several provisions have been made to quality it for both electrically noisy
environments and applications requiring high system security. In such applications unintentional
entering of the power saving modes must be absolutely avoided. A power saving mode would
reduce the controller’s performance (in the case of slow-down mode) or even stop any operation (in
the case of power down mode). This situation might be fatal for the system, which is controlled by
the microcontroller. Such critical applications often use the watchdog timer to prevent the system
from program upsets. Then, an accidental entering of the power saving modes would even stop the
watchdog timer and would circumvent the watchdog timer’s task of system protection.
Semiconductor Group
9-1
1997-10-01