SWRA495L December   2015  – April 2025 CC1310 , CC1350 , CC2620 , CC2630 , CC2640 , CC2640R2F , CC2640R2F-Q1 , CC2642R-Q1 , CC2650 , CC2662R-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Oscillator and Crystal Basics
    1. 1.1 Oscillator Operation
    2. 1.2 Quartz Crystal Electrical Model
      1. 1.2.1 Frequency of Oscillation
      2. 1.2.2 Equivalent Series Resistance
      3. 1.2.3 Drive Level
      4. 1.2.4 Crystal Pulling
    3. 1.3 Negative Resistance
    4. 1.4 Time Constant of the Oscillator
  5. 2 Overview of Crystal Oscillators for CC devices
    1. 2.1 24MHz and 48MHz Crystal Oscillator
    2. 2.2 24MHz and 48MHz Crystal Control Loop
    3. 2.3 32.768kHz Crystal Oscillator
  6. 3 Selecting Crystals for the CC devices
    1. 3.1 Mode of Operation
    2. 3.2 Frequency Accuracy
      1. 3.2.1 24MHz and 48MHz Crystal
      2. 3.2.2 32.768kHz Crystal
    3. 3.3 Load Capacitance
    4. 3.4 ESR and Start-Up Time
    5. 3.5 Drive Level and Power Consumption
    6. 3.6 Crystal Package Size
  7. 4 PCB Layout of the Crystal
  8. 5 Measuring the Amplitude of the Oscillations of Your Crystal
    1. 5.1 Measuring Start-Up Time to Determine HPMRAMP1_TH and XOSC_HF_FAST_START
  9. 6 Crystals for CC13xx, CC26xx, CC23xx and CC27xx
  10. 7 High Performance BAW Oscillator
  11. 8 CC23XX and CC27XX Software Amplitude Compensation
  12. 9 Internal Capacitor Array for CC23XX and CC27XX
  13. 10Internal Capacitor Array for CC13xx and CC26xx
  14. 11Summary
  15. 12References
  16. 13Revision History

1.3 Negative Resistance

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 make sure of sufficient oscillator margin during crystal startup, 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 verified over operating conditions.

Equation 7 approximates the negative resistance and shows that a low CL gives a larger negative resistance.

Equation 7.

where:

    gmis the transconductance of the active element in the oscillator.
    CLis the load capacitance specified in the crystal data sheet.

Note:

For CC23xx and CC27xx, the transconductance (gm) can be approximated as 19 milli-Siemens for the high frequency crystal oscillator during start-up phase.

For CC13xx and CC26xx, the transconductance (gm) can be approximated as 7 milli-Siemens for the high frequency crystal oscillator.

For the low frequency crystal, the transconductance (gm) can be approximated as 30 micro-Siemens.

Users 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.

To make sure of a robust start-up of the crystal oscillator, TI recommends that the magnitude of the negative resistance be at least 10 times greater than the ESR during the initial start-up of the crystal and at least 5 times greater than the ESR during steady-state operation for automotive applications.

While, in certain use cases and applications, these values can be minimized to at least 3 times during start-up, TI recommends to use both initial and steady-state software amplitude compensation found in the SysConfig page. TI does not recommend operating values lower than these. This represents a usage limitation, and proper functionality cannot be verified at these reduced margins.