HT82K70E-L datasheet(28/59 Pages) HOLTEK

28 / 59 page

HT82K70E-L/HT82K70A-L/HT82K76E-L

Rev. 1.20

February 1, 2011

Oscillator

There are two oscillator circuits contained within the de-

vice. The first is the system oscillator which utilises an

external crystal or RC and the second is the Watchdog

timer oscillator which is fully integrated and requires no

external components.

System Clock Configurations

There are two oscillator mode Crystal and RC. For Crys-

tal 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 feedback for oscillation, without requiring external

capacitors. However, for some crystal types and fre-

quencies, to ensure oscillation, it may be necessary to

add two small value capacitors, C1 and C2. Using a ce-

ramic resonator will usually require two small value ca-

pacitors, 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, re-

sistor R1 is not required, R1 may be necessary to en-

sure the oscillator stops running when VDD falls below

its operating range.

External RC Oscillator

Using the external system RC oscillator requires that a

resistor, with a value between 27k

W and 750kW, is con-

nected between OSC1 and VDD, and a capacitor is con-

nected to ground. The generated system clock divided

by 4 will be provided on OSC2 as an output which can

be used for external synchronization purposes. Note

that as the OSC2 output is an NMOS open-drain type, a

pull high resistor should be connected if it to be used to

monitor the internal frequency. Although this is a cost ef-

fective oscillator configuration, the oscillation frequency

can vary with VDD, temperature and process variations

and is therefore not suitable for applications where tim-

ing is critical or where accurate oscillator frequencies

are required.For the value of the external resistor ROSC

refer to the Holtek website for typical RC Oscillator vs.

Temperature and VDD characteristics graphics. Note

that it is the only microcontroller internal circuitry to-

gether with the external resistor, that determine the fre-

quency of the oscillator. The external capacitor shown

on the diagram does not influence the frequency of os-

cillation.

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 65

msat5V

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 ex-

tremely low standby current level. This occurs because

when the device enters the Power Down Mode, the sys-

tem oscillator is stopped which reduces the power con-

sumption 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 applica-

tion 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 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 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.

O S C 1

O S C 2

f S Y S / 4 N M O S O p e n D r a i n

4 7 0 p F

V D D

2 7 k W ~ 7 5 0 k W

RC Oscillator

O S C 2

O S C 1

C 2

C 1

R 1

Crystal/Ceramic Oscillator

HT82K70E-L Datasheet(PDF) 28 Page - Holtek Semiconductor Inc

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