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

2.7.1 Crystal Specifications

When selecting a crystal, keep in mind the respective specifications mentioned in the device's data sheets. Table 2-1 and Table 2-2 show the slow and fast clock respective specifications for only 2.4 GHz Wi-Fi.

Table 2-1 CC31xx and CC32xx RTC Crystal Requirements
Characteristics Test Conditions Min Typ Max Unit
Nominal frequency 32.768 kHz
Frequency accuracy Initial + temperature + aging ±150 ppm
Crystal ESR 32.768 kHz 70 kΩ

Note that low-frequency tuning fork crystals have a resonance frequency that changes with temperature with a parabolic coefficient of (–0.04 × 10e6) / °C2 typically. Figure 2-1 shows an example of this, where it can be seen that a 40-ppm accuracy is maintained only from –10°C to 50°C.

GUID-20200807-CA0I-JWLQ-DJW7-GQHZK9HTBJC3-low.png Figure 2-1 Frequency vs Temperature Curve for a 32.768-kHz Tuning Fork Crystal
Table 2-2 CC31xx and CC32xx 2.4GHz only WLAN Fast-Clock Crystal Requirements
Characteristics Test Conditions Min Typ Max Unit
Nominal frequency 40 MHz
Production Tolerance (Intial Torance) ±10 ppm
Load Capacitance (CL) 8 pF
Temperature Stability ±10 ppm
Aging Assuming 5-year life ±5 ppm
Frequency accuracy Initial + temperature + aging ±25 ppm
Crystal ESR 40 MHz 40 50 60 Ω

Table 2-3 shows the fast clock specifications for 2.4 GHz and 5 GHz Wi-Fi.

Table 2-3 CC31xx and CC32xx for 2.4 GHz and 5GHz WLAN Fast-Clock Crystal Requirements
Characteristics Test Conditions Min Typ Max Unit
Nominal frequency 40 MHz
Production Tolerance ±5 ppm
Load Capacitance (CL) 8 pF
Temperature Stability ±15 ppm
Aging Assuming 5-year life ±3 ppm
Frequency accuracy Initial + temperature + aging ±20 ppm
Crystal ESR 40 MHz 40 Ω
Note: When using devices CC3135 or CC3235 for frequency bandwidth 5 GHz the crystal specifications will differ to than for 2.4Ghz, since more precision is needed.