C508_01 Datasheet, PDF (241/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
Power Saving Modes
9.3
Slow Down Mode Operation
In some applications, where power consumption and dissipation are critical, the
controller might run for a certain time at reduced speed (for example, if the controller is
waiting for an input signal). In CMOS devices, there is an almost linear dependence of
the operating frequency and the power supply current, so, a reduction of the operating
frequency results in reduced power consumption.
The slow down mode is activated by setting the bit SD in SFR PCON. If the slow down
mode is enabled, the clock signals for the CPU and the peripheral units are reduced to
1/32 of the nominal system clock rate. The controller actually enters the slow down mode
after a short synchronization period (maximum of two machine cycles). The slow down
mode is terminated by clearing bit SD.
The slow down mode can be combined with the idle mode by performing the following
double instruction sequence:
ORL PCON,#00000001B ; preparing idle mode: set bit IDLE (IDLS not set)
ORL PCON,#00110000B ; entering idle mode combined with the slow down mode:
; (IDLS and SD set)
There are two ways to terminate the combined Idle and Slow Down Mode:
– The idle mode can be terminated by activation of any enabled interrupt. CPU
operation is resumed, and the interrupt will be serviced. The next instruction to be
executed after the RETI instruction will be the one following the instruction that had
set the bits IDLS and SD. Nevertheless, the slow down mode stays enabled and if
required termination must be done by clearing the bit SD in the corresponding
interrupt service routine or at any point in the program where the user no longer
requires the power saving slow-down mode.
– The other possibility of terminating the combined idle and slow down mode is a
hardware reset. Since the oscillator is still running, the hardware reset must be held
active for only two machine cycles for a complete reset.
User’s Manual
9-5
2001-05