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 PCB Layout Guidelines

The layout of the crystal can reduce the parasitic capacitance and, more importantly, reduce noise from coupling on the input of the oscillators. Noise on the input of the oscillator can lead to severe side effects such as clock glitches, flash corruption, or system crashes because the CC31xx and CC32xx devices rely on the crystal oscillators for the systems fast and slow clock.

The following are a few general recommendations for the layout of the crystals:

  • Place the crystal as close as possible to the device to minimize the length of the PCB traces. (This placement reduces crosstalk and minimizes EMI.)
  • TI recommends a solid ground plane under the crystal.
  • Ensure no high-speed digital signals are close to the crystal to minimize cross-coupling of noise into the oscillator.

Figure 5-1 shows the top layer of the layout of the CC32xx reference design. The bottom layer is a solid ground plane. For more details, see the CC3235S/CC3235SF SimpleLink™ Wi-Fi® LaunchPad™ Design Files, see [12]. The same crystal layout can be used with CC31xx device.

GUID-20200807-CA0I-6ZTB-8SXD-FM38DP4QNLZG-low.png Figure 5-1 Layout of the CC32xx EVM