HT82K74EE datasheet(29/56 Pages) HOLTEK

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HT82K74E/HT82K74EE

Rev. 1.00

December 15, 2009

Oscillator

There are two oscillator circuits contained within the de-

vice. The first is the system oscillator which utilises an

external crystal and the second is the Watchdog timer

oscillator which is fully integrated and requires no exter-

nal components.

System Clock Configurations

There is one oscillator mode Crystal. For Crystal mode

no built-in capacitor between OSC1, OSC2 and GND.

The simple connection of a crystal across OSC1 and

OSC2 will create the necessary phase shift and feed-

back for oscillation, without requiring external capaci-

tors. However, for some crystal types and frequencies,

to ensure oscillation, it may be necessary to add two

small value capacitors, C1 and C2. Using a ceramic res-

onator will usually require two small value capacitors,

C1 and C2, to be connected as shown for oscillation to

occur. The values of C1 and C2 should be selected in

consultation with the crystal or resonator manufacturer's

specification. In most applications, resistor R1 is not re-

quired, however for those applications where the LVR

function is not used, R1 may be necessary to ensure the

oscillator stops running when VDD falls below its operat-

ing range.

More information regarding the oscillator is located in

Application Note HA0075E on the Holtek website.

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 is enabled and the clock source is selected to

come from the WDT oscillator.

· 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 this will continue to run

when in the Power Down Mode and will thus consume

some power.

O S C 2

O S C 1

C 2

C 1

R 1

Crystal/Ceramic Oscillator

HT82K74EE Datasheet(PDF) 29 Page - Holtek Semiconductor Inc

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