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There is a 0.2V -ve voltage at PWM output waveform during OFF period. Why?

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suman.sardar

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In this PWM out put waveform (whic is collected from pin PWM0 of a PIC18F4431) there is a 0.2 volt drop below zero in the pwm output waveform. In the output waveform of the PWM I see that there is a +5v rise during ON period. But also there is a -0.2 volt fall during OFF period. Whts the reason behind this.
Untitled.png
 

We need to see the schematic to be sure but the most likely reasons are your oscilloscope probes are not compensated properly or your ground reference (ground probe) is not at the same potential as the PWM0 pin. The PIC itself can not produce a negative voltage.

Brian.
 

Its shocking to see the -0.2V from PIC output. What is the operating voltage of it? Check for the proper power supply outputs.
 

Hi,

Why shocking?
-0.3V is specified in the datasheet.

Connect a couple meters of wire to a port and see the negative signal every time you switch form HIGH to LOW.

Klaus
 

-0.3V is specified in the datasheet.
Click to expand...
Is it explained in the Data sheet? (Page no?)

Connect a couple meters of wire to a port and see the negative signal every time you switch form HIGH to LOW.
Click to expand...
Whats wire got to do with this?

- - - Updated - - -

Pic Development Board for PCPWM.pdsprj-page-001.jpg
Probe was connected between marked terminals and the ground probe was properly set before connection
 

Hi,

It has nothing to do with ground bounce? (asking, not suggesting) Does it do the same with the probe ground connected to the PIC ground?
 

That isn't ground bounce. If the diagram in post 5 is accurate, I would expect some voltage drop along the ground connection between the bridge driver and the PIC. That would imply the scope wasnt measuring the signal levels produced by the PIC, it was starting from a base level already 0.2V below VSS.

Brian.
 
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Hi,

Is it explained in the Data sheet? (Page no?)
Click to expand...
It is not explained. It it specified...this means the PIC won't get hurt if there are pulses of -0.2V at the IOs.
--> Absolute maximum ratings

Klaus
 

The question I always ask:
Are these waveforms simulated, or are they real oscilloscope waveforms from an actual circuit?

To me, these look like they are simulated. In which case, the reasons for this negative voltage are completely different from the reasons of an actual circuit.
 

Maybe whoever made the macromodel for the PIC
used the min rated voltage level and voltage source
(or "assign") drive, rather than a "switch" output
structure (my preference for a CMOS part).

Be sure that the macromodel properties don't
include a "VOL" that happens to be defaulted
wrong. Do not assume that unloaded VOL=AVSS
and so on. Might test that, static, in another
simulation testbench.
 

Hi,

Whats wire got to do with this?
Click to expand...
Any wire is a complex load with parasitic capacitance and parasitic inductance.
It causes overshot, undershot and ringing.

Klaus
 

This waveform is not simulated. The waveform was plotted in MATLAB with data collected from real hardware and oscilloscope.
 

Hi,

Everything.
If there is the driver circuit, then show it.
If there is anything else, then show it, too.

Klaus
 

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