HT68F03 datasheet(34/147 Pages) HOLTEK

34 / 147 page

HT66F03/HT66F04/HT68F03/HT68F04

Rev. 1.30

February 17, 2011

The actual source clock used for each of the high speed

and low speed oscillators is chosen via configuration

options. The frequency of the slow speed or high speed

system clock is also determined using the HLCLK bit

and CKS2 ~ CKS0 bits in the SMOD register. Note that

two oscillator selections must be made namely one high

speed and one low speed system oscillators. It is not

possible to choose a no-oscillator selection for either the

high or low speed oscillator. The OSC1 and OSC2 pins

are used to connect the external components for the ex-

ternal crystal, external RC and external low speed crys-

tal oscillators. Once the OSC1 or both OSC1 and OSC2

pins are used for ERC or HXT oscillators, the corre-

sponding low speed oscillator must be the Internal Low

speed RC oscillator LIRC. Therefore there are some

limitations to select the high speed and low speed oscil-

lators for the high and low speed system clock sources.

The available selections for high speed and low speed

oscillators are shown in the following table.

Oscillator Types

Available Oscillator

Selections

1234

High Speed Oscillator

HXT

ERC

HIRC

Low Speed Oscillator

LIRC

LXT

External Crystal/ Ceramic Oscillator

- HXT

The External Crystal/ Ceramic System Oscillator is one

of the high frequency oscillator choices, which is se-

lected via configuration option. For most crystal oscilla-

tor configurations, the simple connection of a crystal

across OSC1 and OSC2 will create the necessary

phase shift and feedback for oscillation, without requir-

ing external capacitors. However, for some crystal types

and frequencies, to ensure oscillation, it may be neces-

sary to add two small value capacitors, C1 and C2.

Using a ceramic resonator 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.

Crystal Oscillator C1 and C2 Values

Crystal Frequency

20MHz

6pF

8pF

16MHz

6pF

8pF

12MHz

8pF

10pF

8MHz

8pF

10pF

4MHz

8pF

10pF

1MHz

100pF

Note:

C1 and C2 values are for guidance only.

Crystal Recommended Capacitor Values

External RC Oscillator

- ERC

Using the ERC oscillator only requires that a resistor,

with a value between 56k

W and 2.4MW, is connected

between OSC1 and VDD, and a capacitor is connected

between OSC1 and ground, providing a low cost oscilla-

tor configuration. It is only the external resistor that de-

termines the oscillation frequency; the external

capacitor has no influence over the frequency and is

connected for stability purposes only. Device trimming

during the manufacturing process and the inclusion of

internal frequency compensation circuits are used to en-

sure that the influence of the power supply voltage, tem-

perature and process variations on the oscillation

frequency are minimised. As a resistance/frequency ref-

erence point, it can be noted that with an external 120k

resistor connected and with a 5V voltage power supply

and temperature of 25

°C degrees, the oscillator will

have a frequency of 8MHz within a tolerance of 2%.

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.

Internal RC Oscillator

- HIRC

The internal RC oscillator is a fully integrated system os-

cillator requiring no external components. The internal

RC oscillator has three fixed frequencies of either

4MHz, 8MHz or 12MHz. Device trimming during the

manufacturing process and the inclusion of internal fre-

quency compensation circuits are used to ensure that

the influence of the power supply voltage, temperature

and process variations on the oscillation frequency are

minimised. As a result, at a power supply of 5V and at a

temperature of 25

°C degrees, the fixed oscillation fre-

quency of the high speed internal 8MHz RC oscillator

will have a tolerance within 2%. Note that if this internal

O S C 1

4 7 0 p F

V D D

R O S C

External RC Oscillator

- ERC

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

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 . C 1 a n d C 2 a r e 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

- HXT

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