HT66F40 datasheet(35/246 Pages) HOLTEK

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HT66F20/HT66F30/HT66F40/HT66F50/HT66F60

HT66FU30/HT66FU40/HT66FU50/HT66FU60

Rev. 1.10

February 1, 2010

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.

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

12MHz

0pF

8MHz

0pF

4MHz

0pF

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 PB1, leaving pin PB2 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 either 3V or

5V and at a temperature of 25

°C degrees, the fixed os-

cillation frequency of 4MHz, 8MHz or 12MHz will have a

tolerance within 2%. Note that if this internal system

clock option is selected, as it requires no external pins

for its operation, I/O pins PB1 and PB2 are free for use

as normal I/O pins.

External 32.768kHz Crystal Oscillator

- LXT

The External 32.768kHz Crystal System Oscillator is

one of the low frequency oscillator choices, which is se-

lected via configuration option. This clock source has a

fixed frequency of 32.768kHz and requires a 32.768kHz

crystal to be connected between pins XT1 and XT2. The

external resistor and capacitor components connected

to the 32.768kHz crystal are necessary to provide oscil-

lation. For applications where precise frequencies are

essential, these components may be required to provide

frequency compensation due to different crystal manu-

facturing tolerances. During power-up there is a time

delay associated with the LXT oscillator waiting for it to

start-up.

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

HT66F40 数据表(PDF) 35 Page - Holtek Semiconductor Inc

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