The HF oscillator has internal variable load capacitors (cap-array) in the IC and does not require external capacitors to be mounted. There are some exceptions. For details on when it is required to use external capacitors instead of the internal cap-array, see Section 3.1.1.

The load capacitance is set in CCFG.c through the following defines:

#ifndef SET_CCFG_MODE_CONF_XOSC_CAP_MOD
// #define SET_CCFG_MODE_CONF_XOSC_CAP_MOD            0x0        // Apply cap-array delta
#define SET_CCFG_MODE_CONF_XOSC_CAP_MOD               0x1        // Don't apply cap-array delta 
#endif
#ifndef SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA
#define SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA        0xFF       // Signed 8-bit value, directly modifying trimmed XOSC cap-array value
#endif

The SET_CCFG_MODE_CONF_XOSC_CAP_MOD defines tells the system whether it should use the default value or use an offset from the default value set by SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA. The default cap-array values are 9 pF for CC13x0/CC26x0 QFN, 5 pF for CC2640R2F WCSP, and 6.7 pF for CC13x2/CC26x2.

The cap-array delta value is an offset from the default value that can be either negative or positive. Table 6-1 shows the resulting total capacitance measured on an evaluation board versus cap-array delta values. Note that the resulting capacitance value includes parasitic capacitances, which is why the lowest setting is not 0 pF. Using a delta value equal to or lower than the most negative value in the table completely disables the internal load capacitor array.

The best way to measure the accuracy of the HF crystal oscillator is to output an unmodulated carrier wave from the radio and measuring the frequency offset from the wanted frequency using a spectrum analyzer. The relative offset of crystal frequency, typically stated in Parts per Million (ppm), is the same as the relative offset of the RF carrier.

For testing purposes cap-array delta values can be adjusted in SmartRF™ Studio. This simplifies tuning greatly by allowing on-the-fly updates of the load capacitance. The optimum value found in SmartRF Studio can then be entered into CCFG in the applicable software project.