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

2.1.1 ESR and Start-up Reliability

The crystal ESR is directly related to the oscillator allowance and safety factor. Both the oscillator allowance and safety factor are figures of merit used to establish a level of reliability of the crystal-oscillator start-up. Typically, choosing a low-ESR crystal is done to improve the start-up time and reliability. Conversely, choosing a high-ESR crystal or adding a series resistance within the circuit increases the start-up time and decreases reliability.

The MSP430 MCU data sheet provides the typical values of oscillator allowance for the LFXT1 module shown in Table 2:

Table 2. Oscillator Allowance

PARAMETER TEST CONDITIONS TYP UNIT
OALF Oscillation allowance for LF crystals XTS = 0, LFXT1Sx = 0, fLFXT1,LF = 32768 kHz, CL,eff = 6 pF 500 kΩ
XTS = 0, LFXT1Sx = 0, fLFXT1,LF = 32768 kHz, CL,eff = 12 pF 200

Using equation 5 from MSP430 32-kHz Crystal Oscillators, the change in safety factor by increasing the ESR by 15 to 20 kΩ still results in a safe qualification (see Table 3).

Table 3. Change in Safety Factor

Safety Factor = OA / ESR
Original Safety Factor 200 kΩ / 25 kΩ 8
New Safety Factor 200 kΩ / 45 kΩ 4.4