SWRA680 November   2020 CC3100 , CC3120 , CC3135 , CC3200 , CC3220R , CC3220S , CC3220SF , CC3235S , CC3235SF

 

  1.   Abstract
  2.   Trademarks
  3. 1Crystal Oscillator Basics
    1. 1.1 Crystal Oscillator Model
  4. 2Crystal Selection
    1. 2.1 Crystal Mode of Operation
    2. 2.2 Temperature Tolerance
    3. 2.3 Aging (Long-Term Stability)
    4. 2.4 Crystal ESR
    5. 2.5 Frequency Accuracy
    6. 2.6 Drive Level
    7. 2.7 Selecting a crystal
      1. 2.7.1 Crystal Specifications
      2. 2.7.2 Recommended Crystals for the CC31xx and CC32xx
  5. 3Crystal Tuning
    1. 3.1 The importance of Crystal Tuning
    2. 3.2 Load Capacitance
    3. 3.3 Crystal Tuning With CL
  6. 4Measuring the ppm Error for the Fast Clock (High Frequency)
    1. 4.1 Setting Up for the CC3x00 Devices
    2. 4.2 Setting Up for the CC3x20 and CC3x35 Devices
    3. 4.3 Measuring ppm Frequency Error
  7. 5PCB Layout Guidelines
    1. 5.1 The 32.768-kHz Crystal (Slow Clock)
    2. 5.2 The 40-MHz Crystal (Fast Clock)
  8. 6References

5.1 The 32.768-kHz Crystal (Slow Clock)

This crystal is used as the RTC, which supplies the free-running slow clock. The crystal requirements for the 32.768-kHz crystal oscillator can be seen in Table 2-1.

Below are the PCB gudelines for the slow clock:

  • The 32.768-kHz crystal oscillator is used as the RTC, consequently it should be placed close to the VQFN package.
  • Ensure that the load capacitance is tuned according to the board parasitic capacitance, thus ensuring the frequency tolerance is within ±150 ppm at room temperature, as described in Section 4.
  • The ground plane on layer two is solid below the trace lanes, and there should be ground around these traces on the top layer, as shown in Figure 5-1.