MAX6902(2003) 查看數據表（PDF） - Maxim Integrated
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MAX6902 Datasheet PDF : 18 Pages
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SPI-Compatible RTC in a TDFN
Table 3. 32.768kHz Surface-Mount Watch Crystals
MANUFACTURER
MANUFACTURER
PART NO.
Abracon Corporation
Caliber Electronics
ECS INC International
Fox Electronics
M-tron
Raltron
SaRonix
ABS25-32.768-12.5-B-2-T
AWS2A-32.768KHz, AWS2B-32.768KHz
ECS-.327-12.5-17
FSM327
SX2010/ SX2020
RSE-32.768-12.5-C-T
32S12A
TEMP.
RANGE
-40°C to +85°C
-20°C to +70°C
-10°C to +60°C
-40°C to +85°C
-20°C to +75°C
-10°C to +60°C
-40°C to +85°C
CL (pF)
12.5
12.5
12.5
12.5
12.5
12.5
12.5
+25°C
FREQUENCY
TOLERANCE
(ppm)
±20
±20
±20
±20
±20
±20
±20
k = parabolic curvature constant (-0.035 ±0.005ppm/°C2
for 32.768kHz watch crystals)
T0 = turnover temperature (+25°C ±5°C for 32.768kHz
watch crystals)
T = temperature of interest (°C)
For example: What is the worst-case change in oscilla-
tor frequency from +25°C ambient to +45°C ambient?
∆fdrift = 32.768Hz × (-0.04ppm/oC2 × (1×10-6))
× (20oC - 45oC)2 = -0.8192Hz
What is the worst-case timekeeping error per second?
Error due to temperature drift:
{[ [ ]] } ( ) ∆tdrift = 1/ f + ∆fdrift / 32,768 -1s /1s
{[ [ ]] } ∆tdrift = 1/ (32,768Hz - 0.8192Hz)/ 32,768 -1s /1s
= 0.000025s /s
Error due to 25°C initial crystal tolerance of ±20ppm:
{[ [ ]] } ( ) ∆tinitial = 1/ f + ∆finitial / 32,768 -1s /1s
( ) ∆finitial = 32,768Hz × -20ppm × (1 × 10-6) = 0.65536Hz
Total timekeeping error per second:
{[ [ ]] } ∆tinitial = 1/ (32,768 - 0.65536)/ 32,768 -1 /1s
= 0.000020s /s
∆ttotal = ∆tdrift + ∆tinitial
∆ttotal = 0.000025s /s + 0.000020s /s = 0.000045s /s
TEMPERATURE (°C)
-40 -20 0 20 40 60
0
80 100
-50
-100
-150
-200
-250
TYPICAL TEMPERATURE CHARACTERISTICS
(k = -0.035 ppm/°C2; TO = +25°C)
Figure 6. Typical Temperature Curve for 32.768kHz Watch
Crystal
After 1 month that translates to:
∆t
=
(31day)
24
hr
day
60
min
hr
60
s
min
(0.000045s/s)
=120.528s
Total worst-case timekeeping error at the end of 1
month at 45°C is about 120s or 2min (assumes negligi-
ble parasitic layout capacitance).
Oscillator Start Time
The MAX6902 oscillator typically takes 5s to 10s to
begin oscillating. To ensure the oscillator is operating
correctly, the software should validate proper time-
keeping. This is accomplished by reading the Seconds
register. Any reading of 1s or more from the POR value
of zero seconds is a validation of proper startup.
14 ______________________________________________________________________________________
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