FM3130-G datasheet(5/22 Pages) RAMTRON

5 / 22 page

ire

FM3130 Integrated RTC/Alarm with 64Kb FRAM

Rev. 3.2

Sept. 2011

Page 5 of 22

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

choose to drive X1 with an external clock and X2

with an inverted clock using a CMOS inverter.

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.

Layout for Surface Mount Crystal

Layout for Through Hole Crystal

(red = top layer, green = bottom layer)

Table 2. Calibration Adjustments

Positive Calibration for slow clocks: Calibration will achieve

 2.17 PPM after calibration

Measured Frequency Range

Error Range (PPM)

Min

Max

Min

Max

Program Calibration Register to:

512.0000

511.9989

2.17

000000

511.9989

511.9967

2.18

6.51

100001

511.9967

511.9944

6.52

10.85

100010

511.9944

511.9922

10.86

15.19

100011

511.9922

511.9900

15.20

19.53

100100

511.9900

511.9878

19.54

23.87

100101

511.9878

511.9856

23.88

28.21

100110

511.9856

511.9833

28.22

32.55

100111

511.9833

511.9811

32.56

36.89

101000

511.9811

511.9789

36.90

41.23

101001

511.9789

511.9767

41.24

45.57

101010

511.9767

511.9744

45.58

49.91

101011

511.9744

511.9722

49.92

54.25

101100

511.9722

511.9700

54.26

58.59

101101

511.9700

511.9678

58.60

62.93

101110

511.9678

511.9656

62.94

67.27

101111

511.9656

511.9633

67.28

71.61

110000

511.9633

511.9611

71.62

75.95

110001

511.9611

511.9589

75.96

80.29

110010

511.9589

511.9567

80.30

84.63

110011

511.9567

511.9544

84.64

88.97

110100

511.9544

511.9522

88.98

93.31

110101

511.9522

511.9500

93.32

97.65

110110

511.9500

511.9478

97.66

101.99

110111

Vdd

FM3130

VBAK

VSS

VBAK

VSS

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