HT83F40P datasheet(40/61 Pages) HOLTEK

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HT83FXX

Rev. 1.00

May 12, 2009

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 nor-

mal operating current, will be reduced to an extremely

low standby current level. This occurs because when

the device enters the Power Down Mode, the system

oscillator is stopped which reduces the power consump-

tion 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 requir-

ing a full reset. This feature is extremely important in ap-

plication 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.

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:

· The system oscillator will stop running and the appli-

cation program will stop at the

²HALT² instruction.

· 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 origi-

nates 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 consumption is to be minimized. Special atten-

tion 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 input pins could

create internal oscillations and result in increased cur-

rent 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 current will also

be required if the configuration options have enabled the

Watchdog Timer internal oscillator.

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 de-

vice will experience a full system reset, however, if the

device is woken up by a WDT overflow, a Watchdog

Timer reset will be initiated. Although both of these

wake-up methods will initiate a reset operation, the ac-

tual source of the wake-up can be determined by exam-

ining the TO and PDF flags. The PDF flag is cleared by a

system power-up or executing the clear Watchdog

Timer instructions and is set when executing the

²HALT²

instruction. The TO flag is set if a WDT time-out occurs,

and causes a wake-up that only resets the Program

Counter and Stack Pointer, the other flags remain in

their original status.

Each pin on Port A can be setup via an individual config-

uration option to permit a negative transition on the pin

to wake-up the system. When a Port A pin wake-up oc-

curs, the program will resume execution at the instruc-

tion following the

²HALT² instruction.

If the system is woken up by an interrupt, then two possi-

ble 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 program will resume exe-

cution at the instruction following the

²HALT² instruction.

In this situation, the interrupt which woke-up the device

will not be immediately serviced, but will rather be ser-

viced later when the related interrupt is finally enabled or

when a stack level becomes free. The other situation is

where the related interrupt is enabled and the stack is

not full, in which case the regular interrupt response

takes place. If an interrupt request flag is set to

²1² be-

fore entering the Power Down Mode, the wake-up func-

tion of the related interrupt will be disabled.

No matter what the source of the wake-up event is, once

a wake-up situation occurs, a time period equal to 1024

system clock periods will be required before normal sys-

tem operation resumes. However, if the wake-up has

originated due to an interrupt, the actual interrupt sub-

routine execution will be delayed by an additional one or

more cycles. If the wake-up results in the execution of

the next instruction following the

²HALT² instruction, this

will be executed immediately after the 1024 system

clock period delay has ended.

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