ht82k70e-l Holtek Semiconductor Inc., ht82k70e-l Datasheet - Page 28

• Manufacturer Part Number: ht82k70e-l
• Description: I/o Type 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 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 and 750k , is con-
nected between OSC1 and ground, and a capacitor is
connected to VDD. 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.
Rev. 1.00
Crystal/Ceramic Oscillator
28
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
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:
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 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.
HT82K70E-L/HT82K76E-L
RC Oscillator
September 15, 2009