IEEE 802.15.4 subsystem
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IEEE 802.15.4 subsystem Git Source Tree
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| Source at commit c9e52c1978ed11ef9c404b381131b67703705fbb created 7 years 3 months ago. By Werner Almesberger, atusb/atusb.kicad_pcb: bring back component references | |
|---|---|
| 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 | |
