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

11 Summary

TI’s CC13xx, CC26xx, CC23xx, and CC27xx families of low-power wireless MCUs rely on integrated 24MHz or 48MHz high-frequency oscillators (XOSC-HF or HFXT) as reference clocks for RF and system functions. Accurate clock sources are crucial for maintaining RF performance, regulatory compliance, and efficient system operation. In power-down mode, the high-frequency oscillator is typically off, and a 32.768kHz low-frequency oscillator (XOSC-LF or LFXT) maintains system timing, particularly for time-synchronized protocols like Bluetooth Low Energy. These oscillators use a Pierce architecture, with a crystal and associated capacitors forming a pi filter to stabilize frequency.

Key factors in selecting crystals include equivalent series resistance (ESR), motional inductance, drive level, and capacitive loading. ESR, which represents the crystal’s resistance at resonance, must be low enough to make sure of reliable startup and efficient operation. The crystal’s drive level, defined in microwatts, must not exceed manufacturer specifications to avoid damage. The transconductance (gm) of the oscillator circuit determines the ability to drive the crystal, with CC23xx or CC27xx featuring 19mS gm for high-frequency oscillators, CC13xx or CC26xx at 7mS, and low-frequency oscillators at 30µS. Start-up time depends on negative resistance, motional inductance, and overall circuit conditions.

Frequency accuracy depends on production tolerance, temperature stability, aging, and frequency pulling due to mismatched load capacitance. Load capacitance (CL) is critical for verifying correct oscillation frequency, factoring in both on-chip and external capacitors. Crystals must be chosen to meet device specifications, balancing low-capacitive loading for better power efficiency with stability against environmental variation. To make sure of reliable startup, the negative resistance must be at least three times the ESR. Excessive ESR can lead to longer startup times.

Optimizing crystal selection involves trade-offs between size, performance, and cost. Choosing a crystal with low motional resistance, low motional inductance, and appropriate capacitive loading improves start-up time and reduces power consumption. While a list of recommended crystals is provided, selection is not restricted to those alone, as long as key specifications are met. External capacitors can be used to fine-tune CL if internal capacitance settings are minimized. Proper crystal selection is essential for maintaining stable operation and efficient performance in battery-powered wireless applications.