HT46R01/HT46R02/HT46R03
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Power Down Mode and Wake-up
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Power Down Mode
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All of the Holtek microcontrollers have the ability to enter
a Power Down Mode, also sometimes 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 be-
cause when the device enters the Power Down Mode,
the system oscillator is stopped which reduces the
power consumption to extremely low levels, however,
as the device maintains its present internal condition, it
can be woken up at a later stage and continue running,
without requiring a full reset. This feature is extremely
important in application areas where the MCU must
have its power supply constantly maintained to keep the
device in a known condition but where the power supply
capacity is limited such as in battery applications.
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For applications using the RTC oscillator, the system
clock can be chosen to be either the Internal System RC
Oscillator or the RTC oscillator itself. This selection is
made using the CLKMOD bit in the CTRL0 register. If
this bit is set high then the 32768Hz external crystal will
also provide the system clock source. If the bit is low
then the system clock source will be the Internal RC Os-
cillator. When the system enters the Power Down Mode,
the system clock, irrespective of whether the CLKMOD
bit has selected the RTC or Internal RC Oscillator as its
source, will always stop running. The following table
shows the relationship between the CLKMOD bit and
the various oscillators.
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 ²HALT² instruc-
tion in the application program. When this instruction is
executed, the following will occur:
Operating CLKMOD Internal
System
Clock
RTC
Mode
Bit
RC Osc.
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The system oscillator will stop running and the appli-
0
On
On RC Oscillator
cation program will stop at the ²HALT² instruction.
Normal
Running
If the RTC oscillator configuration option is enabled
then the RTC clock will keep running.
1
Off
On
On
32768Hz
Stopped
Power
Down
X
Off
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
or RTC oscillator. The WDT will stop if its clock source
originates from the system clock.
During power up there is a time delay associated with
the RTC oscillator waiting for it to start up. Bit 1 of the
CTRL0 register, known as the QOSC bit, is provided to
give a quick start-up function and can be used to mini-
mize this delay. During a power up condition, this bit will
be cleared to 0 which will initiate the RTC oscillator quick
start-up function. However, as there is additional power
consumption associated with this quick start-up func-
tion, to reduce power consumption after start up takes
place, it is recommended that the application program
should set the QOSC bit high about 2 seconds after
power on. It should be noted that, no matter what condi-
tion the QOSC bit is set to, the RTC oscillator will always
function normally, only there is more power consump-
tion associated with the quick start-up function.
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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 consumption is to be minimised.
Watchdog Timer Oscillator
Special attention must be made to the I/O pins on the
device. All high-impedance input pins must be con-
nected to either a fixed high or low level as any floating
input pins could create internal oscillations and result in
increased current consumption. Care must also be
taken with the loads, which are connected to I/O pins,
which are setup as outputs. These should be placed in a
condition in which minimum current is drawn or con-
nected only to external circuits that do not draw current,
such as other CMOS inputs.
The WDT oscillator is a fully self-contained free running
on-chip RC oscillator with a typical period of 65ms at 5V
requiring no external components. When the device en-
ters the Power Down Mode, the system clock will stop
running but the WDT oscillator continues to free-run and
to keep the watchdog active. However, to preserve
power in certain applications the WDT oscillator can be
disabled via a configuration option.
Rev. 1.00
42
September 21, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
