HT46R49 datasheet(42/77 Pages) HOLTEK

42 / 77 page

HT46R46/C46/R47/C47/R48A/C48A/R49

Rev. 1.30

June 5, 2008

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

470k

Oscillator Internal Component Values

Crystal Oscillator C1 and C2 Values

Crystal Frequency

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 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.

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

4 7 0 p F

V D D

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

RC Oscillator

HT46R49 数据表(PDF) 42 Page - Holtek Semiconductor Inc

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