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

2.5 Frequency Accuracy

Frequency accuracy represents the maximum deviation of the reference frequency. The total tolerance of the frequency accuracy of a crystal is dependent on several factors:

  • Production tolerance (Tolinitial)
  • Temperature tolerance (Toltemp)
  • Aging effects (Tolage)
  • Frequency pulling of the crystal due to mismatched loading capacitance (Tolpull)

When selecting a crystal consider the mentioned parameters. Equation 1 gives the total crystal tolerance.

Equation 1. GUID-20200807-CA0I-DNDJ-2VD3-PF37GLFBGJ7H-low.gif

The unit ppm, which is an abbreviation of parts per million, is used to describe the accuracy of a frequency. The ppm is a value that represents the part of a whole number in units of 1/1000000 (100 ppm= .01%). The total tolerance value is in parts per million (ppm) and can be found in the crystal data sheet of the crystal manufacturer.

The CC31XX and CC32XX require crystals for a 32.768-kHz and 40.0-MHz frequency with accuracy of ±150 ppm and ±25 ppm, respectively. Section 2.7.1 has the complete crystal specifications.

Note: Frequency pulling may not appear in the crystal data sheet and can be calculated. For more information, see 1.