X1227
APPLICATION SECTION
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
Register, or ATR, which has 6 bits of control. The load
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.
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.
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 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-
Table 6. Crystal Parameters Required for Xicor RTC’s
Parameter
Min
Typ
Max
Units
kHz
Notes
Frequency
32.768
Freq. Tolerance
100
30
ppm
Down to 20ppm if desired
Typically the value used for most
crystals
Turnover Temperature
20
25
°C
Operating Temperature Range
Parallel Load Capacitance
Equivalent Series Resistance
-40
85
°C
pF
kΩ
12.5
50
For best oscillator performance
Table 7. Crystal Manufacturers
Manufacturer
Citizen
Part Number
CM201, CM202, CM200S
MC-405, MC-406
RSM-200S-A or B
32S12A or B
Temp Range
-40 to +85°C
-40 to +85°C
-40 to +85°C
-40 to +85°C
-10 to +60°C
-10 to +60°C
-40 to +85°C
+25°C Freq Toler.
20ppm
Epson
20ppm
Raltron
SaRonix
Ecliptek
ECS
20ppm
20ppm
ECPSM29T-32.768K
ECX-306/ECX-306I
FSM-327
20ppm
20ppm
Fox
20ppm
Characteristics subject to change without notice. 21 of 28
REV 1.1.20 1/13/03
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XICOR [ XICOR INC. ]
