SLAA423A December 2009 – November 2018 MSP430F4132 , MSP430F4152 , MSP430F47126 , MSP430F47127 , MSP430F47163 , MSP430F47166 , MSP430F47167 , MSP430F47173 , MSP430F47176 , MSP430F47177 , MSP430F47183 , MSP430F47186 , MSP430F47187 , MSP430F47193 , MSP430F47196 , MSP430FG4616 , MSP430FG4617 , MSP430FG4618
Erratum XOSC8 for some MSP430™ microcontrollers (MCUs) places an extra consideration upon the crystal oscillator design beyond that found in the crystal oscillator design guide, MSP430 32-kHz Crystal Oscillators. Specifically, the erratum requires that the crystal oscillator circuit provides a minimum level of impedance to force the oscillator circuit of the MSP430 MCU to work harder. This can be done with increased load capacitance, increased ESR, or by placing a resistor from the crystal input to ground.
Each of these workarounds has potential side effects for the crystal-oscillator circuit. A positive side effect is increased noise immunity. The negative side effects include increased power consumption and a decrease in the safety factor. With the decrease in safety factor, the maintenance of an acceptable safety factor becomes more challenging.
Due to the numerous factors that influence the crystal-oscillator circuit, it is not possible to recommend a solution that works in all situations. This application report describes the different components of the crystal oscillator circuit that can be used to mitigate XOSC8, as well as workarounds and the implications of each. The workarounds include choosing a larger ESR crystal and using a shunt resistance on the oscillator input.
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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
VCC and temperature are more related to the occurrence of XOSC8, while the ESR and load capacitance impact both the occurrence of XOSC8 and the oscillation allowance of the crystal-oscillator circuit.