Chapter 1 Hardware Structure
System Clock Source
RTC Oscillator
Clock Source Options
RTC oscillator only
RTC oscillator, WDT oscillator or f
RTC oscillator, WDT oscillator or f
Internal Clock Source Options
Power Down Mode and Wake-up
Power Down Mode
All of the Holtek microcontrollers have the ability to enter a Power Down Mode, also known as the
HALT Mode or Sleep Mode. When the device enters this mode, the normal operating current, will
be reduced to an extremely low standby current level. This occurs because when the device en-
ters the Power Down Mode, the system oscillator is stopped which reduces the power consump-
tion to extremely low levels, however, as the device maintains it present internal condition, it can
be woken up at a later stage and continue running, without requiring a full reset. This feature is ex-
tremely important in application areas where the MCU must have its power supply constantly main-
tained to keep the device in a known condition but where the power supply capacity is limited such
as in battery applications.
Entering the Power Down Mode
There is only one way for the device to enter the Power Down Mode and that is to execute the
instruction in the application program. When this instruction is executed, the following will
The system oscillator will stop running and the application program will stop at the
The Data Memory contents and registers will maintain their present condition.
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. Also note that additional standby cur-
rent will also be required if the configuration options are setup to keep the various Watchdog, RTC
oscillator and LCD functions active during the Power Down Mode.
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