MAX6902ETA-T MAXIM [Maxim Integrated Products], MAX6902ETA-T Datasheet - Page 14

• Manufacturer Part Number: MAX6902ETA-T
• Description: SPI-Compatible RTC in a TDFN
• Manufacturer: MAXIM [Maxim Integrated Products]

SPI-Compatible RTC in a TDFN
Table 3. 32.768kHz Surface-Mount Watch Crystals
k = parabolic curvature constant (-0.035 ±0.005ppm/°C
for 32.768kHz watch crystals)
T
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:
14
Abracon Corporation
Caliber Electronics
ECS INC International
Fox Electronics
M-tron
Raltron
SaRonix
0
∆
∆
t
f
MANUFACTURER
= turnover temperature (+25°C ±5°C for 32.768kHz
∆
∆
∆
initial
∆
∆
initial
t
t
t
______________________________________________________________________________________
t
t
initial
∆f
total
total
drift
drift
drift
=
=
=
=
=
=
=
32 768
=
=
{
=
[
0 000025
{
{
∆
0 000025
1
{
0 000020
[
[
,
.
[
t
1
1 32 768
.
32 768
/
.
1 32 768 0 6
drift
×
[
/
/
(
[
[
/
(
f
[
.
(
(
20
(
f
+
Hz
+
+
∆
o
,
∆
C - 45 C)
,
∆
f
initial
Hz
×
f
t
s s
drift
s s
s s
initial
(
/
/
Hz
-20ppm
/
×
- . 5536
)
)
(
+
o
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
/
/
-0.04ppm/ C
- .8192
0 000020
32 768
32 768
0
.
2
,
,
=
×
-
0 8192
(
]
]
)
.
1 10
]
]
Hz
/
- s
- s
32 768
×
1
1
s s
MANUFACTURER
o
)
/
,
}
/
}
2
32 768
Hz
/
/
-6
1
1
=
PART NO.
× ×
s
s
,
)
0 000045
]
(
)
1 10
]
.
=
-
1 1
0 65536
}
]
]
.
/
- s
-6
1
s
)
}
)
s s
/
1
/
s
Hz
2
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).
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.
Figure 6. Typical Temperature Curve for 32.768kHz Watch
Crystal
∆t
=120.528s
=
(
31
-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
day
-100
-150
-200
-250
-50
RANGE
TEMP.
0
)



-40
TYPICAL TEMPERATURE CHARACTERISTICS
24
(k = -0.035 ppm/°C
day
-20
hr
TEMPERATURE (°C)



0



60
20
min
hr
C
2
Oscillator Start Time
; T O = +25°C)
L
40
12.5
12.5
12.5
12.5
12.5
12.5
12.5



(pF)



60
60
min
s
80



100
(0.000045s/s)
FREQUENCY
TOLERANCE
+25°C
(ppm)
±20
±20
±20
±20
±20
±20
±20