FM3130 Integrated RTC/Alarm with 64Kb FRAM
choose to drive X1 with an external clock and X2
with an inverted clock using a CMOS inverter.
Vdd
FM3130
Layout Recommendations
The X1 and X2 crystal pins employ very high
impedance circuits and the oscillator connected to
these pins can be upset by noise or extra loading. To
reduce RTC clock errors from signal switching noise,
a guard ring should be placed around these pads and
the guard ring grounded. SDA and SCL traces should
be routed away from the X1/X2 pads. The X1 and X2
trace lengths should be less than 5 mm. The use of a
ground plane on the backside or inner board layer is
preferred. See layout example. Red is the top layer,
green is the bottom layer.
R1
R2
X1
Figure 3. External Oscillator
In the example, R1 and R2 are chosen such that the
X2 voltage is centered around the oscillator drive
levels. If you wish to avoid the DC current, you may
E
X1
X1
X2
T
c
X2
t
VBAK
VBAK
n
VSS
e
E
VSS
m
4
L
e
Layout for Surface Mount Crystal
(red = top layer, green = bottom layer)
Layout for Through Hole Crystal
6
(red = top layer, green = bottom layer)
1
a
l
3
p
O
Table 2. Calibration Adjustments
e
M
Positive Calibration for slow clocks: Calibration will achieve 2.17 PPM after calibration
Measured Frequency Range
Error Range (PPM)
F
R
Min
Max
Min
Max
Program Calibration Register to:
S
:
t
0
1
2
3
4
5
6
7
8
512.0000
511.9989
511.9967
511.9944
511.9922
511.9900
511.9878
511.9856
511.9833
511.9989
511.9967
511.9944
511.9922
B
511.9900
511.9878
511.9856
511.9833
0
2.17
000000
2.18
6.52
10.86
15.20
6.51
100001
e
c
10.85
15.19
19.53
23.87
100010
v
e
i
100011
100100
100101
100110
100111
101000
101001
101010
101011
101100
101101
101110
101111
110000
110001
110010
110011
110100
110101
110110
110111
r
i
t
19.54
a
D
23.88
28.21
n
28.22
32.56
36.90
41.24
32.55
r
511.9811
511.9789
o
36.89
41.23
45.57
49.91
54.25
58.59
62.93
67.27
71.61
75.95
80.29
84.63
88.97
93.31
97.65
O
e
t
9
511.9811
511.9789
511.9767
511.9744
511.9722
511.9700
511.9678
511.9656
511.9633
511.9611
511.9589
511.9567
511.9544
511.9522
511.9500
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
511.9767
511.9744
511.9722
511.9700
511.9678
511.9656
511.9633
511.9611
511.9589
511.9567
511.9544
511.9522
511.9500
511.9478
l
45.58
A
49.92
54.26
58.60
62.94
67.28
71.62
75.96
80.30
84.64
88.98
93.32
97.66
101.99
Rev. 3.2
Sept. 2011
Page 5 of 22
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