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Application Report
SLAA322 – August 2006
MSP430 32-kHz Crystal Oscillators
Peter Spevak and Peter Forstner ............................................................................ MSP430 Applications
ABSTRACT
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 MSP430
ultralow-power operation. 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.
Contents
1 The 32-kHz Crystal Oscillator ..................................................................... 2
2 Crystal Selection .................................................................................... 3
3 PCB Design considerations ....................................................................... 6
4 Testing the Crystal Oscillator ..................................................................... 8
5 Crystal Oscillator in Production ................................................................... 9
List of Figures
1 Mechanical Oscillation of a Tuning-Fork Crystal ............................................... 2
2 Equivalent Circuit of a Crystal .................................................................... 2
3 Reactance of a Crystal ............................................................................. 2
4 Principle Pierce Oscillator Circuit ................................................................. 3
5 Frequency vs Load Capacitance for a 0-ppm Crystal ......................................... 4
6 Frequency Deviation of a Tuning-Fork Crystal Over Temperature .......................... 5
7 Layout Without and With External Load Capacitors (XIN and XOUT Neighboring
Pins Are Standard Function Pins) ................................................................ 7
8 Layout With External Capacitors and Ground Guard Ring (XIN and XOUT
Neighboring Pins Are NC Pins) Examples for MSP430F41x and MSP430F1232IRHB .. 7
9 Negative Resistance Method With Added Resistor R
Q
........................................ 9
List of Tables
1 Typical Oscillation Allowance Values for the 32-kHz Oscillator .............................. 5
2 Safety Factor ........................................................................................ 9
SLAA322 – August 2006 MSP430 32-kHz Crystal Oscillators 1
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