AN95089 datasheet(13/19 Pages) RAMTRON

13 / 19 page

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques

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Document No. 001-95089 Rev. *A

Figure 11. External Application View of PSoC 4/PRoC BLE Devices

CL = 6 pF or12.5pF

CL = 6pF, C1=18pF C2=9pF

CL = 12.5pF, C1=37.5 pF C2=18.75pF

Y1=32. 768kHz

P6[1]

P6[0]

+ Cstray

CL =

(C1 xC2)

(C1+C2)

A higher C1/C2 ratio results in a higher current consumption but improved duty cycle.

Based on extensive characterization, a ratio of 2:1 (C1= 2×C2) for the external capacitors is found to be optimal with

respect to the performance and power consumption. The tradeoff for using a 2:1 ratio is slightly higher ICC (Integrated

Chip Current) of up to 100 nA. For a 2:1 ratio, the recommended external capacitor values are

C1 = 37.5 pF, C2 = 18.75 pF (for CL = 12.5 pF)

C1 = 18 pF, C2 = 9 pF (for CL = 6 pF)

8.4

CL and Clock Accuracy

The parasitic capacitance due to trace lines and pads can vary depending on the board layout. You should take into

account the parasitic capacitance of each leg while calculating the load capacitance.

The load capacitance may be further tuned to achieve a better clock accuracy. This can be done as follows:



If the average ppm is negative (i.e., the measured frequency is less than 32.768 kHz), then decrease both C1

and C2 capacitance values, while keeping the ratio of C1 to C2 to within 10 percent of the recommended 2:1 ratio

(a ratio of 1.8 to 2.2 is acceptable if exact capacitance values are not available). However, if the negative ppm

change is very small, then only C2 can be changed.



If the average ppm is positive (i.e., the measured frequency is more than 32.768 kHz), then increase both C1 and

C2 capacitance values while keeping the ratio of C1 to C2 to within 10 percent of the recommended 2:1 ratio (a

ratio of 1.8 to 2.2 is acceptable if exact capacitance values are not available). However, if the positive ppm

change is very small, then only C2 can be changed.

8.5

Frequency Variation Across Boards

Table 3 and Figure 12 show the frequency deviation (ppm) for a WCO measured across different boards having the

same load capacitance values (in the ratio of 2:1). The table also shows how tuning the load capacitance within the

recommended 10 percent variation range further reduces the ppm. Tuning is done only for one board; the same

tuned capacitor values are used for all the boards.

Table 3. ppm Across Boards for 2:1 Cap Ratio

Board

ppm Before Tuning

ppm After Tuning

76.29

56.45

39.36

33.56

52.49

45.77

63.47

54.93

44.55

37.23

64.69

43.94

68.35

50.65

70.19

51.87

62.56

40.58

42.72

34.17

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