HT82K74EE HOLTEK [Holtek Semiconductor Inc], HT82K74EE Datasheet - Page 29

• Manufacturer Part Number: HT82K74EE
• Description: 27MHz Keyboard/ Mouse TX 8-Bit MCU
• Manufacturer: HOLTEK [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 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 s at 3V
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
Rev. 1.00
Crystal/Ceramic Oscillator
29
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 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.
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.
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 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.
HT82K74E/HT82K74EE
December 15, 2009