User`s manual

Semiconductor Group 9-4
Power Saving Modes
C515
The idle mode is entered by two consecutive instructions. The first instruction sets the flag bit IDLE
(PCON.0) and must not set bit IDLS (PCON.5), the following instruction sets the start bit IDLS
(PCON.5) and must not set bit IDLE (PCON.0). The hardware ensures that a concurrent setting of
both bits, IDLE and IDLS, does not initiate the idle mode. Bits IDLE and IDLS will automatically be
cleared after being set. If one of these register bits is read the value that appears is 0. This double
instruction is implemented to minimize the chance of an unintentional entering of the idle mode
which would leave the watchdog timer's task of system protection without effect.
Note:
PCON is not a bit-addressable register, so the above mentioned sequence for entering the idle
mode is obtained by byte-handling instructions, as shown in the following example:
ORL PCON,#00000001B ;Set bit IDLE, bit IDLS must not be set
ORL PCON,#00100000B ;Set bit IDLS, bit IDLE must not be set
The instruction that sets bit IDLS is the last instruction executed before going into idle mode.
There are two ways to terminate the idle mode:
– The idle mode can be terminated by activating any enabled interrupt. The CPU operation is
resumed, the interrupt will be serviced and the next intsruction to be executed after the RETI
instruction will be the one following the instruction that had set the bit IDLS.
– The other way to terminate the idle mode, is a hardware reset. Since the oscillator is still
running, the hardware reset must be held active only for two machine cycles for a complete
reset.