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Low Voltage on crystal

Jiří Babka
Intellectual 465 points
Other Parts Discussed in Thread: MSP430F5419, MSP430F149

Hello,

we have 18.432 MHz crystal connected to the pins XOUT and XIN (port P7) of the microcontroller MSP430F5419. There are capacitors of 33 pF on both side of crystal connected to ground. There is 0.3 V only on XOUT. It seems to us it is too low voltage and therefore the microcontroller is so susceptible to electromagnetic interference.

We was trying to change the capacitance of capacitors near crystal and there is maximum amplitude (0.3 V) for 33 pF on XOUT. We have also changed the crystal to 7.3728 MHz and the problem is the same. We have also the boards with MSP430F149 and there it is ok - the voltage amplitude is more then 2V.

We also tried all combinations of XT1DRIVE in UCSCTL6 register and the voltage amplitude was the best (0.35V) for XT1DRIVE = 01 - it means the range from 8 to 16 MHz - but the crystal was 18.432 MHz.

Could you give us some advice how to increase the voltage amplitude on XOUT? Or is the amplitude of 0.3V normal for MSP430F5419?

Best regards,

Jiri Babka

  • 0
    Guru 227245 points

    The oscillator swings around the hysteresis of the input triggers. It’s an analog oscillation. The maximum amplitude depends on the speed of the oscillator logic. The faster the inverter, the lower the amplitude. 1x family has slow circuitry (8MHz max, even on 3.6V), while 5x family Is much faster (up to 25MHz, so 3 times faster -> 1/3rd of the amplitude).
    Well, that’s just a rough explanation, there are other effects too.

    However, how did you measure the voltage? With a scope? A standard probe? Those probes have a rater low input impedance and also a large input capacitance. You can’t properly measure crystal oscillations with them. In most cases, touching the crystal with a probe will even stop oscillation. For testing our crystals, we bought a quite expensive active FET probe (10M||0.9pF).

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