HT86A72 datasheet(38/68 Pages) HOLTEK

38 / 68 page

HT86Axx/HT86ARxx

Rev. 1.10

December 12, 2012

Oscillator

Various oscillator options offer the user a wide range of

functions according to their various application require-

ments. Two types of system clocks can be selected

while various clock source options for the Watchdog

Timer are provided for maximum flexibility. All oscillator

options are selected through the configuration options.

The two methods of generating the system clock are:

· External crystal/resonator oscillator

· External RC oscillator

One of these two methods must be selected using the

configuration options.

More information regarding the oscillator is located in

Application Note HA0075E on the Holtek website.

External Crystal/Resonator Oscillator

The simple connection of a crystal across OSC1 and

OSC2 will create the necessary phase shift and feed-

back for oscillation, and will normally not require exter-

nal capacitors. However, for some crystals and most

resonator types, to ensure oscillation and accurate fre-

quency generation, it may be necessary to add two

small value external capacitors, C1 and C2. The exact

values of C1 and C2 should be selected in consultation

with the crystal or resonator manufacturer

¢s specifica-

tion. The external parallel feedback resistor, Rp, is nor-

mally not required but in some cases may be needed to

assist with oscillation start up.

Internal Ca, Cb, Rf Typical Values @ 5V, 25

°C

11~13pF

13~15pF

800k

Oscillator Internal Component Values

External RC Oscillator

Using the external system RC oscillator requires that an

external resistor is connected. For the Mask version de-

vice a value of between 120k

W and 280kW is required

For the OPT device a value of between 40k

W and 90kW

is required. This external resistor is connected between

OSC1 and VSS. A clock signal, with a frequency of the

generated system clock divided by 4, will be provided on

OSC2 as an output which can be used for external syn-

chronisation 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 fre-

quency. Although this is a cost effective oscillator con-

figuration, the oscillation frequency can vary with VDD,

temperature and process variations and is therefore not

suitable for applications where timing is critical or where

accurate oscillator frequencies are required. Note that it

is the only microcontroller internal circuitry together with

the external resistor, that determines the frequency of

the oscillator.

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.

RTC Oscillator

A 32KHz crystal can be connected to pins XIN and

XOUT to implement an RTC oscillator. The RTC oscilla-

tor is used as a clock source for Timer/Event Counter 3

but must be first enabled using a configuration option. If

the configuration option enables the RTC oscillator then

it will automatically become the clock source for the

Timer/Event Counter 3. The RTC oscillator will continue

to operate even if the HALT instruction is executed and

the device is powered down. It may be necessary to

connect two small capacitors between XIN, XOUT and

ground for correct operation of the RTC.

O S C 1

O S C 2

R f

T o i n t e r n a l

c i r c u i t s

I n t e r n a l

O s c i l l a t o r

C i r c u i t

C 1

C 2

C a

C b

R p

N o t e : 1 . R p i s n o r m a l l y n o t r e q u i r e d .

2 . A l t h o u g h n o t s h o w n O S C 1 / O S C 2 p i n s h a v e a p a r a s i t i c

c a p a c i t a n c e o f a r o u n d 7 p F .

Crystal/Resonator Oscillator

O S C 1

R O S C

O S C 2

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

External RC Oscillator

X O U T

X I N

3 2 7 6 8 H z

RTC Oscillator

HT86A72 Datasheet(PDF) 38 Page - Holtek Semiconductor Inc

HT86A72 Datasheet(HTML) 38 Page - Holtek Semiconductor Inc

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