Root/ecn/ecn0002.txt

1Add load capacitors to 16 MHz crystal (C14, C15)
2
3
4The crystal has a specified load capacitance of 8 pF. The AT86RF230 has
5an internal capacitor array that can be trimmed in 16 steps from 0 pF to
64.5 pF.
7
8The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature
9tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were
10measured with the CNTR board, roughly at 20-25 C:
11
12Board Error Meas. accuracy C14/C15 Trim
13        (f, ppm) (ppm, nom.) (pF) (pF)
14--------------- --------------- --------------- ------------ --------
152, "orange" +268 99.6 NC 0
16        +266 99.5 NC 0
17        +266 40.0 NC 0
18        +148 99.8 NC 3.0
19        +147 99.7 NC 3.0
20        +147 40.0 NC 3.0
21        +111 100.0 NC 4.5
22        +110 99.8 NC 4.5
23        - 13 99.9 15 0
24        - 14 100.0 15 0
25        + 11 99.9 12 0
26        + 11 99.6 12 0
27(cleanup) + 14 100.0 12 0
28        + 14 100.0 12 0
29
301, "red" +263 99.7 NC 0
31        +263 100.0 NC 0
32        +107 99.3 NC 4.5
33        +108 100.0 NC 4.5
34        + 24 99.8 10 0
35        + 24 100.0 10 0
36        - 9 99.7 10 4.5
37        - 9 100.0 10 4.5
38(cleanup) + 14 100.0 12 0
39        + 14 100.0 12 0
40
41(Multiple measurements to assess CNTR performance and drift. Note that
42CNTR measurements are repeatable within 1 ppm, so the nominal accuracy
43appears to be far too pessimistic.)
44
45
46Conclusion: these measurements suggest that, combined with parasitic
47capacitance, load capacitors of 12 pF make the crystal roughly half the
48trim range faster than 16 MHz.
49

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