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

3.2.2 32.768kHz Crystal

The 32.768kHz crystal oscillator is used as the Read Time Clock (RTC) and is kept running when the device is in Standby mode. Because Bluetooth® low energy is a time-synchronized protocol, an accurate clock also enables longer periods of time in a low-power mode. If a lower-accuracy crystal is used, the device must wake up early to accommodate for the lower accuracy of the clock. To be compliant with Bluetooth® low energy, the clock must have a maximum of ±500 ppm of inaccuracy. For more information, see [2]. TI recommends using a tighter tolerance 32.768kHz crystal to reduce the average power consumption in a typical Bluetooth® low energy connection. In the SimpleLink™ CC2650 EVM Kit 4XD (CC2650EM-4XD) v1.0.3 Design Files, TI uses the Epson FC-135 crystal. If a crystal with different specifications is used, this setting must be adjusted for in the Bluetooth® low energy stack. For more details, see the BLE Stack Users Guide.

Note that low-frequency tuning fork crystals have a resonance frequency that changes with temperature with a parabolic coefficient of (–0.04 × 10– 6) / °C2 typically. Figure 3-1 shows an example. In Figure 3-1, 40ppm accuracy is maintained only from –10°C to 50°C.

Typical Frequency vs Temperature Curve for a 32.768kHz Tuning Fork Crystal Figure 3-1 Typical Frequency vs Temperature Curve for a 32.768kHz Tuning Fork Crystal