C515A_9708 Datasheet, PDF (150/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Power Saving Modes
C515A
9.2 Idle Mode
In the idle mode the oscillator of the C515A continues to run, but the CPU is gated off from the clock
signal. However, the interrupt system, the serial port, the A/D converter,and all timers with the
exception of the watchdog timer are further provided with the clock. The CPU status is preserved in
its entirety: the stack pointer, program counter, program status word, accumulator, and all other
registers maintain their data during idle mode.
The reduction of power consumption, which can be achieved by this feature depends on the number
of peripherals running. If all timers are stopped and the A/D converter, and the serial interfaces are
not running, the maximum power reduction can be achieved. This state is also the test condition for
the idle mode I_CC.
Thus, the user has to take care which peripheral should continue to run and which has to be stopped
during idle mode. Also the state of all port pins – either the pins controlled by their latches or
controlled by their secondary functions – depends on the status of the controller when entering idle
mode.
Normally, the port pins hold the logical state they had at the time when the idle mode was activated.
If some pins are programmed to serve as alternate functions they still continue to output during idle
mode if the assigned function is on. This especially applies to the system clock output signal at pin
P1.6/CLKOUT and to the serial interfaces in case it cannot finish reception or transmission during
normal operation. The control signals ALE and PSEN are hold at logic high levels.
As in normal operation mode, the ports can be used as inputs during idle mode. Thus a capture or
reload operation can be triggered, the timers can be used to count external events, and external
interrupts will be detected.
The idle mode is a useful feature which makes it possible to "freeze" the processor's status - either
for a predefined time, or until an external event reverts the controller to normal operation, as
discussed below. The watchdog timer is the only peripheral which is automatically stopped during
idle mode.
Semiconductor Group
9-3
1997-08-01