ht82k73e Holtek Semiconductor Inc., ht82k73e Datasheet - Page 23

• Manufacturer Part Number: ht82k73e
• Description: 2.4ghz Keyboard Tx 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

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 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, however for those applications
where the LVR function is not used, R1 may be neces-
sary to ensure the oscillator stops running when VDD
falls below its operating range.
For RC mode use pull-high IP (the application add R be-
tween OSC1 and V
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 s 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
Rev. 1.00
Crystal/Ceramic Oscillator
DD
RC Oscillator
).
23
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 HALT instruc-
tion in the application program. When this instruction is
executed, the following will occur:
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.
The system oscillator will stop running and the appli-
cation program will stop at the HALT 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, will be set
and the Watchdog time-out flag, TO, will be cleared.
HT82K73E
April 16, 2008