Add load capacitors to 16 MHz crystal (C14, C15) The crystal has a specified load capacitance of 8 pF. The AT86RF230 has an internal capacitor array that can be trimmed in 16 steps from 0 pF to 4.5 pF. The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were measured with the CNTR board, roughly at 20-25 C: Board Error Meas. accuracy C14/C15 Trim (f, ppm) (ppm, nom.) (pF) (pF) --------------- --------------- --------------- ------------ -------- 2, "orange" +268 99.6 NC 0 +266 99.5 NC 0 +266 40.0 NC 0 +148 99.8 NC 3.0 +147 99.7 NC 3.0 +147 40.0 NC 3.0 +111 100.0 NC 4.5 +110 99.8 NC 4.5 - 13 99.9 15 0 - 14 100.0 15 0 + 11 99.9 12 0 + 11 99.6 12 0 (cleanup) + 14 100.0 12 0 + 14 100.0 12 0 1, "red" +263 99.7 NC 0 +263 100.0 NC 0 +107 99.3 NC 4.5 +108 100.0 NC 4.5 + 24 99.8 10 0 + 24 100.0 10 0 - 9 99.7 10 4.5 - 9 100.0 10 4.5 (cleanup) + 14 100.0 12 0 + 14 100.0 12 0 (Multiple measurements to assess CNTR performance and drift. Note that CNTR measurements are repeatable within 1 ppm, so the nominal accuracy appears to be far too pessimistic.) Conclusion: these measurements suggest that, combined with parasitic capacitance, load capacitors of 12 pF make the crystal roughly half the trim range faster than 16 MHz.