SLAA423A December   2009  – November 2018 MSP430F4132 , MSP430F4152 , MSP430F47126 , MSP430F47127 , MSP430F47163 , MSP430F47166 , MSP430F47167 , MSP430F47173 , MSP430F47176 , MSP430F47177 , MSP430F47183 , MSP430F47186 , MSP430F47187 , MSP430F47193 , MSP430F47196 , MSP430FG4616 , MSP430FG4617 , MSP430FG4618

 

  1.   XOSC8 Guidance
    1.     Trademarks
    2. 1 Introduction
    3. 2 Contribution of ESR, Load Capacitance, VCC, and Temperature
      1. 2.1 Crystal ESR
        1. 2.1.1 ESR and Start-up Reliability
        2. 2.1.2 ESR Specification
      2. 2.2 Load Capacitance
      3. 2.3 Temperature and VCC
    4. 3 Using a Shunt Resistor From XIN to GND
    5. 4 Failsafe Mechanisms
      1. 4.1 2xx Family
      2. 4.2 4xx Family
    6. 5 Summary
    7. 6 References
  2.   Revision History

2.2 Load Capacitance

A larger load capacitance value significantly reduces the probability of a failure even if the ESR condition is not met and VCC and temperature are at the worst case conditions. The failure rate for test crystal 3a, at -40°C, with 0-pF, 6-pF, and 10-pF loads was 17%, 15%, and 2% respectively. When the load capacitance was increased to 12.5 pF, the failure rate became 0%. Similarly, the failure rate was 0% when crystal 3b was tested with a 10-pF load (see Table 4).

Table 4. Crystal Parameters for Crystal 3a, 3b

Test Crystal fs
(Hz)		 FL(nom)
(Hz)		 RM
(Ω)		 LM
(H)		 CM
(fF)		 C0
(pF)		 CL(nom)
(pF)
3a (Citizen) 32761.488 32768.000 26753.600 8506.470 2.774 1.416 5.563
3b (Micro Crystal) 32765.367 32768.000 34247.900 10816.200 2.181 1.061 12.511

Increasing the load capacitance beyond the recommended loading of the crystal results in a frequency error. This is further described in MSP430 LFXT1 Oscillator Accuracy. Also see the crystal manufacturer's data sheet.

It is highly recommended to use at least the minimum load capacitance for the crystal. The load capacitance settings used in this testing are the internal load settings of the MSP430 MCU. External capacitance can be used to achieve the same loading. Using too small or no load capacitance is not recommended. Using both internal and external load capacitors is also not recommended.