HT45R37 datasheet(62/100 Pages) HOLTEK

62 / 100 page

HT45R37

Rev. 1.10

January 29, 2010

Oscillator

Various oscillator options offer the user a wide range of

functions according to their various application require-

ments. Five 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 six methods of generating the system clock,

three high oscillators, two low oscillators and an exter-

nally supplied clock. The three high oscillators are the

external crystal/ceramic oscillator, internal RC oscillator

and the external RC network. The two low oscillators are

the fully integrated 32K_INT oscillator and the external

32768Hz oscillator. Selecting whether the low or high

oscillator is used as the system oscillator is imple-

mented using the HLCLK bit in the CLKMOD register.

The source clock for the high and low oscillators is cho-

sen via configuration options. The frequency of the slow

oscillator

also

determined

using

the

SLOWC0~SLOWC2 bits in the CLKMOD register.

System Crystal/Ceramic Oscillator

After selecting the external crystal configuration option,

the simple connection of a crystal across OSC1 and

OSC2, is normally all that is required to create the nec-

essary 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. In most applica-

tions, resistor R1 is not required, however for those ap-

plications where the LVR function is not used, R1 may

be necessary to ensure the oscillator stops running

when VDD falls below its operating range. The internal

oscillator circuit contains a filter circuit to reduce the

possibility of erratic operation due to noise on the oscil-

lator pins. An additional configuration option must be

setup to configure the device according to whether the

oscillator frequency is high, defined as equal to or above

1MHz, or low, which is defined as below 1 MHz.

More information regarding oscillator applications is lo-

cated 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 47k

W and 1.5MW,iscon-

nected between OSC1 and VDD, and a 470pF capacitor

is connected to ground. Although this is a cost effective

oscillator 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 an internal capacitor together with the external

resistor, ROSC, are the components which determine the

frequency of the oscillator. The external capacitor

shown on the diagram does not influence the frequency

of oscillation. Note that if this external system RC oscil-

lation option is selected, as it requires OSC1 external

pin for its operation, the PC2/OSC2 pin is free for use as

normal I/O pin.The internal oscillator circuit contains a

filter circuit to reduce the possibility of erratic operation

due to noise on the oscillator pins.

Internal RC Oscillator

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, the choice of which is indicated

by the configuration options. Note that if this internal

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

nal pins for its operation, the OSC1 and OSC2 pins are

free for use as normal I/O pins. Refer to the Appendix

section for more information on the actual internal oscil-

lator frequency vs. Temperature and VDD characteris-

tics graphics.

O S C 2

O S C 1

C 2

C 1

R 1

Crystal/Ceramic Oscillator

O S C 1

P C 2

4 7 0 p F

V D D

R O S C

RC Oscillator

HT45R37 Datasheet(PDF) 62 Page - Holtek Semiconductor Inc

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