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

3.3 Crystal Tuning With CL

To achieve the specified frequency from a crystal, the net capacitance load should exactly match the specified load capacitance, CL, of the crystal. If the crystal specifies CL = 8 pF, then each capacitor on the crystal pin would be 16 pF (equal to 2 x 8 pF) as the capacitors would be in series. Out of this 16 pF, account for 2 to 3 pF for the board parasitic. Then, the CC31xx/CC32xx device input has 5 to 6 pF per pin. This calls for a total added capacitor of 7-9 pF on each pin. Fine tune the CL based on the transmitted (TX) output frequency.

For example when using the recommended crystal “Q24FA20H00396”, with specified CL of 8 pF, it would resonate at 40 Mhz accurately only when the net capacitance between the “XI Pin” and “XO Pin” pins is accurately 8 pF.

Keep in mind that the capacitance moves away from 8 pF in temperature changes, thus the frequency error will also change. A typical plot indicating the ppm error with variation in CL is shown in Figure 3-2.

GUID-20200807-CA0I-FFBR-BCNJ-TMJM2ZXWDMBZ-low.png Figure 3-2 Frequency Error vs. Load Capacitance
Note: Never tune the crystal by monitoring the crystal itself. Loading the crystal with an oscilloscope probe will reduce the frequency of oscillation.