SWRA495K December   2015  – April 2024 CC1310 , CC1350 , CC2620 , CC2630 , CC2640 , CC2640R2F , CC2640R2F-Q1 , CC2642R-Q1 , CC2650 , CC2662R-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Oscillator 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. 2Overview of Crystal Oscillators for CC devices
    1. 2.1 24-MHz and 48-MHz Crystal Oscillator
    2. 2.2 24-MHz and 48-MHz Crystal Control Loop
    3. 2.3 32.768-kHz Crystal Oscillator
  6. 3Selecting Crystals for the CC devices
    1. 3.1 Mode of Operation
    2. 3.2 Frequency Accuracy
      1. 3.2.1 24-MHz and 48-MHz Crystal
      2. 3.2.2 32.768-kHz 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. 4PCB Layout of the Crystal
  8. 5Measuring 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. 6Crystals for CC13xx, CC26xx and CC23xx
  10. 7High Performance BAW Oscillator
  11. 8References
  12. 9Revision History

3.2.2 32.768-kHz Crystal

The 32.768-kHz 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 32.768-kHz 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 of this . In Figure 3-1, 40-ppm accuracy is maintained only from –10°C to 50°C.

GUID-CB8AF08C-05CA-47D8-B04E-9B3FDFA701A4-low.gif Figure 3-1 Typical Frequency vs Temperature Curve for a 32.768-kHz Tuning Fork Crystal