HT37B90 datasheet(43/80 Pages) HOLTEK

43 / 80 page

HT37B90/HT37B70/HT37B50/HT37B30

Rev.1.00

June22,2017

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

optionsareselectedthroughtheconfigurationoptions.

Thetwomethodsofgeneratingthesystemclockare:

· Externalcrystal/resonatoroscillator

· External RCoscillator

One of these two methods must be selected using the

configurationoptions.

More information regarding the oscillator is located in

ApplicationNoteHA0075Eonthe Holtek website.

ExternalCrystal/ResonatorOscillator

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

assistwithoscillationstartup.

InternalCa, Cb,RfTypicalValues@5V,25°C

7pF~9pF

9pF~11pF

300k

OscillatorInternalComponentValues

ExternalRCOscillator

Using the external system RC oscillator requires that a

resistor, with a value between 82k

W and 180kW, is con-

nected between OSC1 and VSS. The generated system

clock divided by 8 will be provided on OSC2 as an output

which can be used for external synchronization pur-

poses. 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 effective oscillator configuration, the oscilla-

tion frequency can vary with VDD, temperature and pro-

cess variations and is therefore not suitable for

applications where timing is critical or where accurate os-

cillator frequencies are required. For the value of the ex-

ternal resistor. Note that it is the only microcontroller

internal circuitry together with the external resistor, that

determine the frequency of the oscillator. The external

capacitor shown on the diagram does not influence the

frequency of oscillation.

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/ResonatorOscillator

O S C 1

R O S C

O S C 2

f O S C / 8 N M O S O p e n D r a i n

ExternalRCOscillator

HT37B90 Datasheet(PDF) 43 Page - Holtek Semiconductor Inc

HT37B90 Datasheet(HTML) 43 Page - Holtek Semiconductor Inc

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