MSP430™ 32-kHz Crystal Oscillators
MSP430™ 32-kHz Crystal Oscillators
Selection of the right crystal, correct load circuit, and proper board layout are important for a stable crystal oscillator. This application report summarizes crystal oscillator function and explains the parameters to select the correct crystal for ultra-low-power operation of an MSP430™ MCU. In addition, hints and examples for correct board layout are given. The document also contains detailed information on the possible oscillator tests to ensure stable oscillator operation in mass production.
For additional information about the factors that influence the accuracy of the low-frequency oscillator, see MSP430™ LFXT1 Oscillator Accuracy.
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MSP430 is a trademark of Texas Instruments.
3M, Novec are trademarks of 3M.
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1 The 32-kHz Crystal Oscillator
1.1 The Crystal
For an ultra-low-power design, only low-frequency crystals are usable, because with higher-frequency oscillators, the current consumption increases significantly. Tuning-fork crystals typically have a frequency range of 10 kHz to 200 kHz in fundamental mode and a maximum drive level of 1 µW. These parameters make them the first choice for the 32768-Hz ultra-low-power crystal oscillator in MSP430 microcontrollers.
Every MSP430 MCU has a built-in crystal oscillator that can be operated with a tuning-fork crystal at 32768 Hz (often called 32 kHz). The mechanical oscillation (see Figure 1) of a 32-kHz tuning fork crystal is converted into an electrical signal. The equivalent electrical circuit of a crystal (see Figure 2) gives these electrical characteristics:
- CM motional capacitance
- LM motional inductance
- RM mechanical losses during oscillation
- C0 parasitic capacitance of package and pins
Figure 1. Mechanical Oscillation of a Tuning-Fork Crystal 
Figure 2. Equivalent Circuit of a Crystal The series-resonance circuit consisting of CM, LM, and RM represents the electrical equivalent of the mechanical resonance of the tuning fork. The frequency characteristics of a crystal's reactance are shown in Figure 3 and give two special frequencies:
- FS (series resonance frequency) solely depends on CM and LM and gives a very stable frequency value.
- FA (anti-resonance or parallel-resonance frequency), in addition, also depends on C0, the parasitic capacitance of package and pins, which is not as precise as the other parameters, CM and LM. Hence, FA gives a less well-defined frequency than FS.
Figure 3. Reactance of a Crystal The equivalent series resistance (ESR) can be calculated with the formula in Equation 1 from the equivalent circuit in Figure 2:
C0 is shown in Figure 2 and given by the crystal's data sheet, as is RM or ESR. CL is the required load capacitance of a crystal and is also given by the crystal's data sheet.