32kHz clock

[Rajinder]

Hi all,
I wanted to ask a few questions regarding measuring a clock frequency of 32.768kHz.

This is connected to a PIC micro with capacitors (15pF) to each side of the crystal, other side to gnd. Also have a series 0R resistor in the xtal in pin.


1. Capacitance probe will cause loading, so would a low capacitance probe be beneficial? I ask because i thought that at low frequencies the capacitive reactance would be high. Hence a x10 probe would be ok.

2. Should I see 32kHz on the xtal in and out pins of my PIC micro? I am seeing 32kHz on one side but 11kHz on the other side of the crystal.

3. Why would a x10 probe not be good enough for measuring this clock.

Thanks in advance.

 

[AnalogKid]

A generic answer for most microcontrollers; your results may vary:

If you look at the clock section of the PIC datasheet, it should show a simplified version of the internal schematic for the oscillator. One side of the crystal is driven by an amplifier or gate output. This is the pin to measure; it has a lower impedance and will be less affected by the probe characteristics.

ak

 

[Rajinder]

A generic answer for most microcontrollers; your results may vary:

If you look at the clock section of the PIC datasheet, it should show a simplified version of the internal schematic for the oscillator. One side of the crystal is driven by an amplifier or gate output. This is the pin to measure; it has a lower impedance and will be less affected by the probe characteristics.

ak

Thanks, but should i see 32kHz on input and output or only output? Should the output be larger in amplitude?
Thanks in advance