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

1 Introduction

The LFXT1 OSC circuit regulates the amount of energy supplied to the crystal-oscillator circuit. This regulation provides the smallest amount of energy to the circuit that still maintains oscillation. The benefits are to provide more energy at start-up to improve reliability and to reduce the amount of energy that maintains the oscillation during operation.

The energy associated with the oscillator circuit is directly related to the energy provided to the comparator, which converts the analog oscillation into the digital clock LFXT1. When the energy supplied to the oscillator is decreased, the energy is also decreased to the comparator. If the decrease is large enough, then the comparator does not recognize a valid analog input. The crystal-oscillator circuit is still functional, but the coupling between the analog circuit and the digital clock (LFXT1) is broken. This can be typically seen as a failure to meet the 30% duty cycle that the data sheet specifies for ACLK or the stopping of ACLK. This bug is referred to as XOSC8:

XOSC8 LFXT1 Module
Function ACLK failure when crystal ESR is below 40 kΩ
Description When ACLK is sourced by a low-frequency crystal with an ESR below 40 kΩ, the duty cycle of ACLK may fall below the specification, the OFIFG may become set or, in some instances, ACLK may stop completely.
Workaround Use a crystal with an ESR greater than 40 kΩ.

The performance of the comparator is affected by the temperature, VCC, and the energy required for oscillation. The amount on energy required for oscillation is impacted by the board layout, the crystal ESR, and the load capacitance seen by the oscillator. As a reference point, the discussion in this document is based upon a board layout that is in accordance with MSP430 32-kHz Crystal Oscillators. With the board layout being 'held constant', the other parameters are varied to show the impact of each. The worst corner case is low temperature, high VCC, low ESR, and low load capacitance. ESR and load capacitance are the most easily controlled by the designer and are the basis for the workarounds provided in this application report