+ /* xtal_trim value is calculated by:
+ * CL = 0.5 * (CX + CTRIM + CPAR)
+ *
+ * whereas:
+ * CL = capacitor of used crystal
+ * CX = connected capacitors at xtal pins
+ * CPAR = in all at86rf2xx datasheets this is a constant value 3 pF,
+ * but this is different on each board setup. You need to fine
+ * tuning this value via CTRIM.
+ * CTRIM = variable capacitor setting. Resolution is 0.3 pF range is
+ * 0 pF upto 4.5 pF.
+ *
+ * Examples:
+ * atben transceiver:
+ *
+ * CL = 8 pF
+ * CX = 12 pF
+ * CPAR = 3 pF (We assume the magic constant from datasheet)
+ * CTRIM = 0.9 pF
+ *
+ * (12+0.9+3)/2 = 7.95 which is nearly at 8 pF
+ *
+ * xtal_trim = 0x3
+ *
+ * openlabs transceiver:
+ *
+ * CL = 16 pF
+ * CX = 22 pF
+ * CPAR = 3 pF (We assume the magic constant from datasheet)
+ * CTRIM = 4.5 pF
+ *
+ * (22+4.5+3)/2 = 14.75 which is the nearest value to 16 pF
+ *
+ * xtal_trim = 0xf
+ */
+ rc = at86rf230_write_subreg(lp, SR_XTAL_TRIM, xtal_trim);
+ if (rc)
+ return rc;
+