HT46R92 datasheet(39/61 Pages) HOLTEK

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HT46R92

Rev. 1.10

November 5, 2008

Oscillator

Various oscillator options offer the user a wide range of

functions according to their various application require-

ments. Three types of system clocks can be selected

while various clock source options for the Watchdog

Timer are provided for maximum flexibility.

The Three methods of generating the system clock are:

· External crystal/resonator oscillator

· External RC oscillator

· External 32768Hz oscillator

The choice of External crystal/resonator or RC system

oscillator is made via a configuration option while the se-

lection of the 32768Hz oscillator is made using the

MODS bit in the MODE register. The 32768Hz Oscillator

must always be connected along with a choice of either

RC or crystal/resonator for correct operation.

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

11pF~13pF

13pF~15pF

470k

Oscillator Internal Component Values

Crystal Oscillator C1 and C2 Values

Crystal Frequency

12MHz

TBD

8MHz

TBD

4MHz

TBD

1MHz

TBD

Note:

1. C1 and C2 values are for guidance only.

2. CL is the crystal manufacturer specified

load capacitor value.

Crystal Recommended Capacitor Values

Resonator C1 and C2 Values

Resonator Frequency

3.58MHz

TBD

1MHz

TBD

455kHz

TBD

Note:

C1 and C2 values are for guidance only.

Resonator Recommended Capacitor Values

External RC Oscillator

Using the external system RC oscillator requires that a

resistor, with a value between 24k

W and 1MW, is con-

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

nected to ground. Although this is a cost effective

oscillator configuration, 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 re-

quired. For the value of the external resistor ROSC refer

to the Holtek website for typical RC Oscillator vs. Tem-

perature and VDD characteristics graphics Here only

the OSC1 pin is used, which is shared with I/O pin PA6,

leaving pin PA5 free for use as a normal I/O pin. 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.

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 .

External Crystal/Ceramic Oscillator

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

R O S C

External RC Oscillator

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