HT46R46E datasheet(46/73 Pages) HOLTEK

46 / 73 page

HT46R46E/C46E/R47E/C47E/R48AE/C48AE/R49E

Rev. 1.20

August 15, 2007

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

options are selected through the configuration options.

System Clock Configurations

There are two methods of generating the system clock,

using an external crystal/ceramic oscillator or an exter-

nal RC network. The chosen method is selected through

the configuration options.

System Crystal/Ceramic Oscillator

After selecting the correct oscillator configuration op-

tion, for most crystal oscillator configurations, the simple

connection of a crystal across OSC1 and OSC2 will cre-

ate the necessary phase shift and feedback for oscilla-

tion, without requiring external capacitors. However, for

some crystal types and frequencies, to ensure oscilla-

tion, it may be necessary to add two small value capaci-

tors, 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. In

most applications, resistor R1 is not required, however

for those applications where the LVR function is not

used, R1 may be necessary to ensure the oscillator

stops running when VDD falls below its operating range.

More information regarding the oscillator is located in

Application Note HA0075E on the Holtek website.

External System RC Oscillator

After selecting the correct configuration option, using

the external system RC oscillator requires that a resis-

tor, with a value between 24k

W and 1MW, is connected

between OSC1 and VDD, and a 470pF capacitor is con-

nected to ground. Although this is a cost effective oscil-

lator configuration, the oscillation frequency can vary

with VDD, temperature and process variations and is

therefore not suitable for applications where timing is

critical or where accurate oscillator frequencies are re-

quired. For the value of the external resistor ROSC refer

to the Appendix section for typical RC Oscillator vs.

Temperature and VDD characteristics graphics.

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. The external capacitor is added to improve

oscillator stability, especially if the open-drain OSC2

output is utilised in the application circuit.

O S C 2

O S C 1

C 2

C 1

R 1

Crystal/Ceramic Oscillator

O S C 1

O S C 2

f S Y S / 4 N M O S O p e n D r a i n

4 7 0 p F

V D D

R O S C

RC Oscillator

HT46R46E Datasheet(PDF) 46 Page - Holtek Semiconductor Inc

HT46R46E Datasheet(HTML) 46 Page - Holtek Semiconductor Inc

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