ISL12028_1011 Datasheet, PDF (23/29 Pages) Intersil Corporation – Real Time Clock/Calendar with I2C Bus™ and EEPROM

ISL12028, ISL12028A
PARAMETER
Frequency
Frequency Tolerance
Turnover-Temperature
Operating Temperature Range
Parallel Load Capacitance
Equivalent Series Resistance
TABLE 9. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTC
MIN
TYP
MAX
UNITS
NOTES
32.768
kHz
±100
ppm
Down to 20ppm if desired
20
25
30
°C
Typically the value used for most crystals
-40
85
°C
12.5
pF
50
kΩ
For best oscillator performance
MANUFACTURER
Citizen
Epson
Raltron
SaRonix
Ecliptek
ECS
Fox
TABLE 10. CRYSTAL MANUFACTURERS
PART NUMBER
CM201, CM202, CM200S
MC-405, MC-406
RSM-200S-A or B
32S12A or B
ECPSM29T-32.768K
ECX-306/ECX-306I
FSM-327
TEMP RANGE
(°C)
-40 to +85
-40 to +85
-40 to +85
-40 to +85
-10 to +60
-10 to +60
-40 to +85
+25°C FREQUENCY
TOLERANCE
(ppm)
±20
±20
±20
±20
±20
±20
±20
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_OUT output is set at 32.768kHz and
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.
Layout Considerations
The crystal input at X1 has a very high impedance and will
pick up high frequency signals from other circuits on the
board. Since the X2 pin is tied to the other side of the crystal,
it is also a sensitive node. These signals can couple into the
oscillator circuit and produce double clocking or misclocking,
seriously affecting the accuracy of the RTC. Care needs to
be taken in layout of the RTC circuit to avoid noise pickup.
Figure 27 shows a suggested layout for the ISL12029 or
ISL12028 devices (R1 is not needed for the ISL12028).
.
C_C1₁
0^..¹1^µµ^FF
X^XT^TAA^LL¹1
3³²2^..⁷7⁶6⁸8^kk^HG^zz
R_R1_{1 1}1₀0_kk
U^U1¹
IS_XL₁1₂2₂0₈28
FIGURE 27. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-14
23
FN8233.9
November 30, 2010