HT68F03 datasheet(29/139 Pages) HOLTEK

29 / 139 page

HT66F03/HT66F04/HT68F03/HT68F04

Rev. 1.00

April 16, 2010

system clock option is selected, as it requires no exter-

nal pins for its operation, I/O pins PA6 and PA5 are free

for use as normal I/O pins or external 32.768 kHz crystal

oscillator (LXT) 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 OSC1 and OSC2.

The external resistor and capacitor components con-

nected to the 32.768kHz crystal are necessary to pro-

vide oscillation. For applications where precise

frequencies are essential, these components may be

required to provide frequency compensation due to dif-

ferent crystal manufacturing tolerances. During

power-up there is a time delay associated with the LXT

oscillator waiting for it to start-up.

When the microcontroller enters the SLEEP or IDLE

Mode, the system clock is switched off to stop

microcontroller activity and to conserve power. How-

ever, in many microcontroller applications it may be nec-

essary to keep the internal timers operational even

when the microcontroller is in the SLEEP or IDLE Mode.

To do this, another clock, independent of the system

clock, must be provided.

However, for some crystals, to ensure oscillation and

accurate frequency generation, it is necessary to add

two small value external capacitors, C1 and C2. The ex-

act values of C1 and C2 should be selected in consulta-

tion with the crystal or resonator manufacturer

¢s

specification. The external parallel feedback resistor,

Rp, is required.

Some configuration options determine if the OSC1 and

OSC2 pins are used for the LXT oscillator or as I/O pins.

· If the LXT oscillator is not used for any clock source,

the OSC1 and OSC2 pins can be used as normal I/O

pins.

· If the LXT oscillator is used for any clock source, the

32.768kHz crystal should be connected to the OSC1

and OSC2 pins.

LXT Oscillator C1 and C2 Values

Crystal Frequency

32.768kHz

10pF

Note:

1. C1 and C2 values are for guidance only.

2. RP=5M~10M

W is recommended.

32.768kHz Crystal Recommended Capacitor Values

LXT Oscillator Low Power Function

The LXT oscillator can function in one of two modes, the

Quick Start Mode and the Low Power Mode. The mode

selection is executed using the LXTLP bit in the TBC

LXTLP Bit

LXT Mode

Quick Start

Low-power

After power on the LXTLP bit will be automatically

cleared to zero ensuring that the LXT oscillator is in the

Quick Start operating mode. In the Quick Start Mode the

LXT oscillator will power up and stabilise quickly. How-

ever, after the LXT oscillator has fully powered up it can

be placed into the Low-power mode by setting the

LXTLP bit high. The oscillator will continue to run but

with reduced current consumption, as the higher current

consumption is only required during the LXT oscillator

start-up. In power sensitive applications, such as battery

applications, where power consumption must be kept to

a minimum, it is therefore recommended that the appli-

cation program sets the LXTLP bit high about 2 seconds

after power-on.

It should be noted that, no matter what condition the

LXTLP bit is set to, the LXT oscillator will always func-

tion normally, the only difference is that it will take more

time to start up if in the Low-power mode.

Internal 32kHz Oscillator

- LIRC

The Internal 32kHz System Oscillator is one of the low

frequency oscillator choices, which is selected via con-

figuration option. It is a fully integrated RC oscillator with

a typical frequency of 32kHz at 5V, requiring no external

components for its implementation. 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 result, at a power supply

of 5V and at a temperature of 25

°C degrees, the fixed

oscillation frequency of 32kHz will have a tolerance

within 10%.

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 , 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 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 .

3 2 . 7 6 8

k H z

O S C 1

O S C 2

External LXT Oscillator

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