Holtek HT48R05A-1, Руководство пользователя, Страница: 47 / 125

·
The WDT will be cleared and resume counting if the WDT clock source is selected to come from
the WDT oscillator. The WDT will stop if its clock source originates from the system clock.
·
The I/O ports will maintain their present condition.
·
In the status register, the Power Down flag, PDF, will be set and the Watchdog time-out flag, TO,
will be cleared.
Standby Current Considerations
As the main reason for entering the Power Down Mode is to keep the current consumption of the
MCU to as low a value as possible, perhaps only in the order of several micro-amps, there are
other considerations which must also be taken into account by the circuit designer if the power con-
sumption is to be minimized. Special attention must be made to the I/O pins on the device. All
high-impedance input pins must be connected to either a fixed high or low level as any floating in-
put pins could create internal oscillations and result in increased current consumption. Care must
also be taken with the loads which are connected to I/Os which are setup as outputs. These
should be placed in a condition in which minimum current is drawn or connected only to external
circuits that do not draw current such as other CMOS inputs.
Wake-up
After the system enters the Power Down Mode, it can be woken up from one of various sources
listed as follows:
·
An external reset
·
An external falling edge on Port A
·
A system interrupt
·
A WDT overflow
If the system is woken up by an external reset, the device will experience a full system reset, how-
ever, if the device is woken up by a WDT overflow, a Watchdog Timer reset will be initiated. Al-
though both of these wake-up methods will initiate a reset operation, the actual source of the
wake-up can be determined by examining the TO and PDF flags. The PDF flag is cleared by a sys-
tem power-up or executing the clear Watchdog Timer instructions and is set when executing the
²
HALT
²
instruction. The TO flag is set if a WDT time-out occurs, and causes a wake-up that only re-
sets the Program Counter and Stack Pointer, the other flags remain in their original status.
If the system is woken up by an interrupt, then two possible situations may occur. The first is where
the related interrupt is disabled or the interrupt is enabled but the stack is full, in which case the pro-
gram will resume execution at the instruction following the
²
HALT
²
instruction. In this situation, the
interrupt which woke-up the device will not be immediately serviced, but will rather be serviced
later when the related interrupt is finally enabled or when a stack level becomes free. The other sit-
uation is where the related interrupt is enabled and the stack is not full, in which case the regular in-
terrupt response takes place. If an interrupt request flag is set to 1 before entering the Power
Down Mode, the wake-up function of the related interrupt will be disabled.
No matter what the source of the wake-up event, once a wake-up situation occurs, a time period
equal to 1024 system clock periods will be required before normal system operation resumes.
However, if the wake-up has originated due to an interrupt, the actual interrupt subroutine execu-
tion will be delayed by an additional one or more cycles. If the wake-up results in the execution of
the next instruction following the
²
HALT
²
instruction, this will be executed immediately after the
1024 system clock period delay has ended.
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Cost-Effective I/O Type MCU