X1227 datasheet(21/28 Pages) XICOR

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X1227

REV 1.1.20 1/13/03

Characteristics subject to change without notice.

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APPLICATION SECTION

CRYSTAL OSCILLATOR AND TEMPERATURE

COMPENSATION

Xicor has now integrated the oscillator compensation

circuity on-chip, to eliminate the need for external com-

ponents and adjust for crystal drift over temperature

and enable very high accuracy time keeping (<5ppm

drift.

The Xicor RTC family uses an oscillator circuit with on-

chip crystal compensation network, including adjust-

able load-capacitance. The only external component

required is the crystal. The compensation network is

optimized for operation with certain crystal parameters

which are common in many of the surface mount or

tuning-fork crystals available today. Table 6 summa-

rizes these parameters.

Table 7 contains some crystal manufacturers and part

numbers that meet the requirements for the Xicor RTC

products.

The turnover temperature in Table 6 describes the

temperature where the apex of the of the drift vs. tem-

perature curve occurs. This curve is parabolic with the

drift increasing as (T-T0)2. For an Epson MC-405

device, for example, the turnover temperature is typi-

cally 25 deg C, and a peak drift of >110ppm occurs at

the temperature extremes of –40 and +85 deg C. It is

possible to address this variable drift by adjusting the

load capacitance of the crystal, which will result in pre-

dictable change to the crystal frequency. The Xicor

RTC family allows this adjustment over temperature

since the devices include on-chip load capacitor trim-

ming. This control is handled by the Analog Trimming

capacitance range covered by the ATR circuit is

approximately 3.25pF to 18.75pF, in 0.25pf incre-

ments. Note that actual capacitance would also

include about 2pF of package related capacitance. In-

circuit tests with commercially available crystals dem-

onstrate that this range of capacitance allows fre-

quency control from +116ppm to –37ppm, using a

12.5pF load crystal.

In addition to the analog compensation afforded by the

adjustable load capacitance, a digital compensation

feature is available for the Xicor RTC family. There are

three bits known as the Digital Trimming Register or

DTR, and they operate by adding or skipping pulses in

the clock signal. The range provided is ±30ppm in

increments of 10ppm. The default setting is 0ppm. The

DTR control can be used for coarse adjustments of

frequency drift over temperature or for crystal initial

accuracy correction.

Table 6. Crystal Parameters Required for Xicor RTC’s

Table 7. Crystal Manufacturers

Parameter

Min

Typ

Max

Units

Notes

Frequency

32.768

kHz

Freq. Tolerance

±100

ppm

Down to 20ppm if desired

Turnover Temperature

°C

Typically the value used for most

crystals

Operating Temperature Range

-40

°C

Parallel Load Capacitance

12.5

Equivalent Series Resistance

Ω

For best oscillator performance

Manufacturer

Part Number

Temp Range

+25°C Freq Toler.

Citizen

CM201, CM202, CM200S

-40 to +85°C

±20ppm

Epson

MC-405, MC-406

-40 to +85°C

±20ppm

Raltron

RSM-200S-A or B

-40 to +85°C

±20ppm

SaRonix

32S12A or B

-40 to +85°C

±20ppm

Ecliptek

ECPSM29T-32.768K

-10 to +60°C

±20ppm

ECS

ECX-306/ECX-306I

-10 to +60°C

±20ppm

Fox

FSM-327

-40 to +85°C

±20ppm

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