X1286_06 Datasheet, PDF (20/25 Pages) Intersil Corporation – Intersil Real Time Clock/Calendar/CPU Supervisor with EEPROM

X1286
APPLICATION SECTION
CRYSTAL OSCILLATOR AND TEMPERATURE
COMPENSATION
Intersil has now integrated the oscillator compensation
circuity on-chip, to eliminate the need for external
components and adjust for crystal drift over tempera-
ture and enable very high accuracy time keeping
(<5ppm drift).
The Intersil RTC family uses an oscillator circuit with
on-chip crystal compensation network, including
adjustable load-capacitance. The only external com-
ponent required is the crystal. The compensation net-
work 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
summarizes these parameters.
Table 7 contains some crystal manufacturers and part
numbers that meet the requirements for the Intersil
RTC products.
The turnover temperature in Table 7 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 Intersil
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.
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation
feature is available for the Intersil 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. Crystal Parameters Required for Intersil RTC’s
Parameter
Min
Typ
Max
Frequency
32.768
Freq. Tolerance
±100
Turnover Temperature
20
25
30
Units
kHz
ppm
°C
Operating Temperature Range
-40
85
°C
Parallel Load Capacitance
12.5
pF
Equivalent Series Resistance
50
kΩ
Table 8. Crystal Manufacturers
Manufacturer
Part Number
Citizen
CM201, CM202, CM200S
Epson
Raltron
MC-405, MC-406
RSM-200S-A or B
SaRonix
32S12A or B
Ecliptek
ECS
ECPSM29T-32.768K
ECX-306/ECX-306I
Fox
FSM-327
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
Notes
Down to 20ppm if desired
Typically the value used for most
crystals
For best oscillator performance
+25°C Freq Toler.
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
20
FN8101.1
April 14, 2006