IEEE 802.15.4 subsystem
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Source at commit c3ac7d9cb8ff9909ad82a2ff952a25b7312408af created 13 years 1 month ago. By Werner Almesberger, atusb/fw3/README: build instructions for firmware and prerequisites | |
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1 | Add load capacitors to 16 MHz crystal (C14, C15) |
2 | |
3 | |
4 | The crystal has a specified load capacitance of 8 pF. The AT86RF230 has |
5 | an internal capacitor array that can be trimmed in 16 steps from 0 pF to |
6 | 4.5 pF. |
7 | |
8 | The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature |
9 | tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were |
10 | measured with the CNTR board, roughly at 20-25 C: |
11 | |
12 | Board Error Meas. accuracy C14/C15 Trim |
13 | (f, ppm) (ppm, nom.) (pF) (pF) |
14 | --------------- --------------- --------------- ------------ -------- |
15 | 2, "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 | |
30 | 1, "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 |
42 | CNTR measurements are repeatable within 1 ppm, so the nominal accuracy |
43 | appears to be far too pessimistic.) |
44 | |
45 | |
46 | Conclusion: these measurements suggest that, combined with parasitic |
47 | capacitance, load capacitors of 12 pF make the crystal roughly half the |
48 | trim range faster than 16 MHz. |
49 |