C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 201

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

9.3
In some applications, where power consumption and dissipation are critical, the controller might run
for a certain time at reduced speed (e.g. if the controller is waiting for an input signal). Since in
CMOS devices there is an almost linear dependence of the operating frequency and the power
supply current, a reduction of the operating frequency results in reduced power consumption.
In the slow down mode all signal frequencies that are derived from the oscillator clock are divided
by 32.
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 (max. 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:
There are two ways to terminate the combined Idle and Slow Down Mode :
Semiconductor Group
– The idle mode can be terminated by activation of any enabled interrupt. The CPU operation
– The other possibility of terminating the combined idle and slow down mode is a hardware
ORL
ORL
is resumed, the interrupt will be serviced and 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 keeps enabled and if required has to be terminated 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 slow-down mode power saving.
reset. Since the oscillator is still running, the hardware reset has to be held active for only two
machine cycles for a complete reset.
Slow Down Mode Operation
PCON,#00000001B
PCON,#00110000B
; preparing idle mode: set bit IDLE (IDLS not set)
; entering idle mode combined with the slow down mode:
; (IDLS and SD set)
9-5
Power Saving Modes
C505 / C505C
1997-08-01