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

1.4 Time Constant of the Oscillator

The start-up time of a crystal oscillator is determined by transient conditions at turn-on, small-signal envelope expansion due to negative resistance, and large-signal amplitude limiting. The envelope expansion is a function of the total negative resistance and the motional inductance of the crystal. The time constant of the envelope expansion is proportional to the start-up time of the oscillator given by Equation 8.

Equation 8.

A crystal with a low LM gives a shorter start-up time and so does a high-magnitude RN (low CL). A trade-off exists between pull-ability due to low-motional capacitance (CM) and fast start-up time due to low-motional inductance (LM), because the frequency of the crystal is dependent on the both CM and LM. Crystals in smaller package sizes have larger LM, and start more slowly than those in larger package sizes (see Section 1.2.1).

Table 1-1 summarizes crystal parameters and the values for the reference crystals recommended by TI for use with the CC devices.

Table 1-1 Crystal Parameters
ParametersDescription24MHz Crystal
Used in TI CC26x0 Characterization		TI-Characterization CC23XX
32.768kHz Crystal		TI-Characterization CC27XX-Q1
32.768kHz Crystal		TI-Characterization CC27XX-Q1
48MHz Crystal
Motional Inductance (LM)Partly determines crystal response time (how quickly the crystal responds to a change from the oscillator). Lower Lm → crystal responds more quickly to changes from the oscillator. Along with CM, a major determiner of the crystal quality factor12.6 mH

3.69

kH

2.95 kH

3.30 mH

Motional Capacitance (CM)Partly determines crystal response time. Lower CM → crystal responds more slowly to changes from the oscillator.3.4 fF6.4 fF

8 fF

3.40fF

Motional Resistance (RM)At resonance, Lm and CM cancel and RM is presented to the oscillator. RM ≃ ESR assuming CL >> CO.20Ω (60Ω maximum)

120

kΩ

max

70 to 75

kΩ		30Ω
Load Capacitance (CL)The amount of load capacitor to tune the crystal to the correct frequency. This load capacitance also helps determine drive level.9pF7pF

7pF

7pF
Shunt Capacitance (C0)This is a parasitic capacitance due to crystal packaging and helps determine the acceptable drive level.1.2pF1.3pF

1.5pF

0.84

pF
ESREquivalent Series Resistance. If CL >> CO, then ESR ≃ RM20Ω (60Ω maximum)

120

kΩ

max

70 to 75

kΩ		30Ω
Drive LevelThe maximum level of power in the crystal for reliable long-term operation, see Equation 5200 µW<500 uW<500 uW50 µW

Typical to 200 µW max