ISL12024 Intersil Corporation, ISL12024 Datasheet - Page 20

• Manufacturer Part Number: ISL12024
• Description: Real-Time Clock/Calendar
• Manufacturer: Intersil Corporation

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 temperature 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 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 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 6 describes the
temperature where the apex of the of the drift vs.
temperature curve occurs. This curve is parabolic with the
drift increasing as (T-T0)
example, the turnover temperature is typically +25°C, and a
peak drift of >110ppm occurs at the temperature extremes of
-40 and +85°C. It is possible to address this variable drift by
adjusting the load capacitance of the crystal, which will result
in predictable change to the crystal frequency. The Intersil
RTC family allows this adjustment over temperature since
the devices include on-chip load capacitor trimming. This
control is handled by the Analog Trimming Register, or ATR,
Frequency
Frequency Tolerance
Turnover Temperature
Operating Temperature Range
Parallel Load Capacitance
Equivalent Series Resistance
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
MANUFACTURER
PARAMETER
2
CM201, CM202, CM200S
MC-405, MC-406
RSM-200S-A or B
32S12A or B
ECPSM29T-32.768K
ECX-306/ECX-306I
FSM-327
. For an Epson MC-405 device, for
20
TABLE 6. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTCs
PART NUMBER
MIN
-40
20
TABLE 7. CRYSTAL MANUFACTURERS
32.768
TYP
12.5
25
ISL12024
MAX
±100
30
85
50
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 increments. Note that actual capacitance
would also include about 2pF of package related
capacitance. In-circuit tests with commercially available
crystals demonstrate that this range of capacitance allows
frequency control from +80ppm to -34ppm, 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.
A final application for the ATR control is in-circuit calibration
for high accuracy applications, along with a temperature
sensor chip. Once the RTC circuit is powered up with battery
backup, the IRQ/F
frequency drift is measured. The ATR control is then
adjusted to a setting which minimizes drift. Once adjusted at
a particular temperature, it is possible to adjust at other
discrete temperatures for minimal overall drift, and store the
resulting settings in the EEPROM. Extremely low overall
temperature drift is possible with this method. The Intersil
evaluation board contains the circuitry necessary to
implement this control.
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
UNITS
ppm
kHz
k
°C
°C
pF
Ω
OUT
Down to 20ppm if desired
Typically the value used for most crystals
For best oscillator performance
output is set at 32.768kHz and
+25°C FREQUENCY TOLERANCE
NOTES
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
±20ppm
October 18, 2006
FN6370.1