SWRA495K December 2015 – April 2024 CC1310 , CC1350 , CC2620 , CC2630 , CC2640 , CC2640R2F , CC2640R2F-Q1 , CC2642R-Q1 , CC2650 , CC2662R-Q1
Negative resistance (RN) is a parameter of the complete oscillator circuit, including capacitor values, crystal parameters, and the on-chip circuit. The CC devices dynamically adjust the oscillator parameters to ensure sufficient oscillator margin during crystal startup and then relax the margins for steady state to decrease the current consumption. This means that when using a crystal within the requirements outlined in the CC data sheets, proper start-up and steady-state margin is ensured over operating conditions.
Equation 7 approximates the negative resistance and shows that a low CL gives a larger negative resistance.
where:
You can also find the negative resistance of the circuit by introducing a resistor in series with the crystal. To avoid parasitic effects, TI recommends using a 0201 resistor for this task. The threshold of the sum of the extra 0201 external resistance and ESR or the crystal where the oscillator is unable to start up is approximately the same as the circuit negative resistance.