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I am sending out the PB clock (32MHz which is PLL multiplied) generated from the internal FRC on a pin to feed into an ADC as a master clock after probing it at 32MHz. This is what I see.

It doesn't look like a pulse in anyway but the duty cycle and frequency almost match up.

Could anyone tell me the possible cause of this issue?

Is it going affect the conversion data rate?

I have also snapped the PCB trace here just in case it could be capacitance from there.

PS: I have traces on both lines and I had an intention of using a stand alone crystal oscillator but totally messed up with the footprint. Luckily I had this alternative.

  • 1
    \$\begingroup\$ You say it's a clock and it looks like a clock...? I don't get the question. Also please have in mind that this kind of fast clock signals are hard to measure accurately with an ordinary scope since the probe adds a bit of capacitance. \$\endgroup\$
    – Lundin
    Feb 12 at 7:15
  • \$\begingroup\$ Do you expect sharper edges? Your MCU is not able to drive a line with sub nano seconds rise- and falltime. But that's not an issue, there is most certainly a clock recovery circuit inside the ADC chip. \$\endgroup\$
    – po.pe
    Feb 12 at 7:38
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    \$\begingroup\$ could anyone tell me the possible cause of this issue - you need to say what the issue is. \$\endgroup\$
    – Andy aka
    Feb 12 at 7:39
  • \$\begingroup\$ Compare the frequency with the AC bandwidth of your scope. That determines how many harmonics of a perfect square clock waveform will make it onto the display. If the scope BW covers only the fundamental and 3rd, it won't look much like a square wave... \$\endgroup\$ Feb 12 at 14:31

You need to look at the port driving capabilities of your MPU (you didn't say which are using) and the requirements of your ADC converter. It almost seems that it's not an high impedance input but some low-impedance or current mode signal (clocks are often transmitted in this way).

Most probably your GPIO is overloaded, or it's configured in a low power mode. As for the effect on the conversion in you ADC sheet there are info on the required levels and duty cycle for correct operation.

Need more info on the MPU and ADC to say more.

What is the result after opening the trace?


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