Max6902 spi-compatible rtc in a tdfn – Rainbow Electronics MAX6902 User Manual
Page 14
MAX6902
SPI-Compatible RTC in a TDFN
14
______________________________________________________________________________________
k = parabolic curvature constant (-0.035 ±0.005ppm/°C
2
for 32.768kHz watch crystals)
T
0
= 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?
What is the worst-case timekeeping error per second?
Error due to temperature drift:
Error due to 25°C initial crystal tolerance of ±20ppm:
Total timekeeping error per second:
After 1 month that translates to:
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.
∆t
day
hr
day
hr
s
=
(
)
31
24
60
60
min
min
(0.000045s/s)
=120.528s
∆
∆
∆
∆
∆
t
s
s s
t
t
t
t
s s
s s
s s
initial
total
drift
initial
total
=
(
)
[
]
[
]
{
}
=
=
+
=
+
=
1 32 768 0 6
32 768
1 1
0 000020
0 000025
0 000020
0 000045
/
,
/
,
/
.
/
.
/
.
/
.
/
- . 5536
-
∆
∆
∆
t
f
f
s
f
Hz
Hz
initial
initial
initial
=
+
(
)
[
]
[
]
{
}
=
Ч
Ч Ч
(
)
=
1
32 768
1
1
32 768
1 10
0 65536
/
/
,
/
,
(
)
.
- s
-20ppm
-6
∆
∆
∆
t
f
f
s
t
Hz
Hz
s
s s
drift
drift
drift
=
+
(
)
[
]
[
]
{
}
=
(
)
[
]
[
]
{
}
=
1
32 768
1
1
1 32 768
0
32 768
1
1
0 000025
/
/
,
/
/
,
/
,
/
.
/
- s
- .8192
- s
∆f
Hz
Hz
drift
=
Ч
Ч Ч
Ч
32 768
1 10
20
0 8192
.
(
)
(
.
(
)
-0.04ppm/ C
C - 45 C)
-
2
-6
2
=
o
o
o
Table 3. 32.768kHz Surface-Mount Watch Crystals
MANUFACTURER
MANUFACTURER
PART NO.
TEMP.
RANGE
C
L
(pF)
+25
°C
FREQUENCY
TOLERANCE
(ppm)
Abracon Corporation
ABS25-32.768-12.5-B-2-T
-40
°C to +85°C
12.5
±20
Caliber Electronics
AWS2A-32.768KHz, AWS2B-32.768KHz
-20
°C to +70°C
12.5
±20
ECS INC International
ECS-.327-12.5-17
-10
°C to +60°C
12.5
±20
Fox Electronics
FSM327
-40
°C to +85°C
12.5
±20
M-tron
SX2010/ SX2020
-20
°C to +75°C
12.5
±20
Raltron
RSE-32.768-12.5-C-T
-10
°C to +60°C
12.5
±20
SaRonix
32S12A
-40
°C to +85°C
12.5
±20
-200
-250
-100
-150
-50
0
-40
-20
0
20
40
60
80
100
TYPICAL TEMPERATURE CHARACTERISTICS
(k = -0.035 ppm/
°C
2
; TO = +25°C)
TEMPERATURE (
°C)
f (ppm)
Figure 6. Typical Temperature Curve for 32.768kHz Watch
Crystal