HT45F23 datasheet(36/152 Pages) HOLTEK

36 / 152 page

Selecting whether the low or high speed oscillator is used as the system oscillator is implemented

using the HLCLK bit and CKS2 ~ CKS0 bits in the SMOD register and as the system clock can be

dynamically selected. 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 selected via configuration option. For most crystal oscillator configurations, the simple

connection of a crystal across OSC1 and OSC2 will create the necessary phase shift and feedback for

oscillation, without requiring external capacitors. However, for some crystal types and frequencies, to

ensure oscillation, it may be necessary 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

Rev. 1.20

September 15, 2011

HT45F23/HT45F43

8-Bit Flash MCU with Op Amps & Comparators

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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