HT82K75REW datasheet(30/103 Pages) HOLTEK

30 / 103 page

HT82M75REW/HT82K75REW

Rev. 1.00

June 11, 2010

Oscillator

The clock source for these devices is provided by an in-

tegrated oscillator requiring no external components.

This oscillator has one fixed frequencies of 6MHz.

Watchdog Timer Oscillator

The WDT oscillator is a fully self-contained free running

on-chip RC oscillator with a typical period of 71

msat3V

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.

Power Down Mode and Wake-up

Power Down Mode

All of the Holtek microcontrollers have the ability to enter

a Power Down 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 microcontroller

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 appli-

cations.

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 function is enabled.

· The I/O ports will maintain their present condition.

· In the status register, the Power Down flag, 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 microcontroller

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 de-

signer if the power consumption is to be minimised.

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.

If the configuration option has enabled the Watchdog

Timer internal oscillator, then the Watchdog Timer will

continue to run when in the Power Down Mode and will

thus consume some power.

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 or rising edge on any of the I/O pins

· A system interrupt

· A WDT overflow (if the contents of the PTR are zeros)

· A PTR overflow occurs (if the contents of the PTR are

not equal to zeros)

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. Note that the WDT time-out will not

occur if the contents of the Period Timer Register (PTR)

are not equal to zeros.

Each pin on Port A or any nibble on other ports can be

setup via configuration options to permit a negative or

positive transition on the pin to wake-up the system.

When a port pin wake-up occurs, the program will re-

sume execution at the instruction 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 interrupt

is disabled or the interrupt is enabled but the stack is full,

in which case the program will resume execution at the

instruction following the

²HALT² instruction. In this situa-

tion, the interrupt will not be immediately serviced, but

will rather be serviced later when the related interrupt is

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