HT56R67 datasheet(77/104 Pages) HOLTEK

77 / 104 page

HT56R67/HT56R668/HT56C668/HT56R678/HT56C678/HT56R688

Rev. 1.20

March 4, 2010

Oscillator

Various oscillator options offer the user a wide range of

functions according to their various application require-

ments. Three types of system clocks can be selected

while various clock source options are provided for max-

imum flexibility. All oscillator options are selected

through the configuration options.

System Clock Configurations

There are three methods of generating the system clock,

two high oscillators and an externally supplied clock. The

two high oscillators are the external crystal/ceramic oscil-

lator and the external RC network. Selecting whether the

low frequency or high oscillator is used as the system os-

cillator is implemented using the HLCLK bit in the

CLKMOD register. The source clock for the high oscillator

is chosen via configuration option. The frequency of the

slow

clock

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 RP1 is not required, however for those ap-

plications where the LVR function is not used, RP1 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.

Crystal Oscillator C1 and C2 Values

Crystal Frequency

12MHz

¾¾

8MHz

¾¾

4MHz

¾¾

1MHz

¾¾

455kHz (see Note 2)

10pF

Note:

1. C1 and C2 values are for guidance only.

2. XTAL mode configuration option: 455kHz.

3. RP1=5M

W~10MW is recommended.

Crystal Recommended Capacitor Values

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. This external capacitor should be added to

improve oscillator stability if the open-drain OSC2 out-

put is utilised in the application circuit. The internal oscil-

lator circuit contains a filter circuit to reduce the

possibility of erratic operation due to noise on the oscil-

lator pins.

Internal 32K_INT Oscillator

When microcontrollers enter a power down condition,

their internal clocks are normally switched off to stop

microcontroller activity and to conserve power. How-

ever, in many microcontroller applications it may be nec-

essary to keep some internal functions operational,

O S C 2

O S C 1

C 2

C 1

R P 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

HT56R67 Datasheet(PDF) 77 Page - Holtek Semiconductor Inc

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