Microcontroller's output signal measurement

Thread Starter

Steven01

Joined Oct 3, 2016
4
Hello,

I have some control board equipped with a microcontroller.
This control board is supplied by 24V laboratory voltage source. I
would like to measure signals on some microcontroller's
output pin with a digital oscilloscope. I have only two common
probes with earthed alligator jack connector. I am not sure
whether the measurement with a probe connected to the output
pin and alligator jack connector connected to minus pole of the
voltage source is correct. Would be better to use both probes
with their alligator jacks connected together and then connect
the first probe on the microcontroller's output pin, the second
probe to the minus pole of the voltage source and use ChA-ChB
function? Thanks.
 

jayanthd

Joined Jul 4, 2015
945
An Oscilloscope will have a probe and in the probe there will be actually two probes. The longer probe is for +ve and the shorter probe of the same cable is for -ve. Connect the longer probe to microcontroller pin and the shorter probe to the ground (in single power supply -ve will be ground) in dual (+ve and -ve power supplies -ve is not ground).

If the microcontroller pin you are measuring is toggling then you will see a pulse (0 to 5V) on the scope.
 

Thread Starter

Steven01

Joined Oct 3, 2016
4
Thank you for your answer Jayanthd.
I supply the control board from a floating
laboratory voltage source. Please can you
tell me where to connect the ground of the
oscilloscope's probe in this situation
(-ve is not ground)? I would like to apologize
to my silly question but I am an autodidact
and I haven't yet fully understand floating
measurements. Thanks.
 

AlbertHall

Joined Jun 4, 2014
12,345
I am not sure
whether the measurement with a probe connected to the output
pin and alligator jack connector connected to minus pole of the
voltage source is correct.
This will work fine as long as the PSU negative is earthed or there is no earth connection. It will only be a problem if some part of the circuit or the positive PSU connection is earthed.
 

jayanthd

Joined Jul 4, 2015
945
If your power supply provides both =ve and -ve voltages like +5V and -5V then -ve is not ground. If your PSU gives only +ve voltages like +5V, +30V then -ve is ground (0V). To measure the voltage at the pin of a microcontroller in +ve only power supply you connect the bigger probe to MCU pin and -ve (smaller probe of the same cable) to the -ve of the circuits power supply.
 

AlbertHall

Joined Jun 4, 2014
12,345
connect the bigger probe to MCU pin and -ve (smaller probe of the same cable) to the -ve of the circuits power supply.
I don't recognise this description of a 'scope probe. This picture shows what I know as a 'scope probe. It has a grounding clip and the probe tip itself (in this case a clip) and this is the kind described by the TS.
 

Thread Starter

Steven01

Joined Oct 3, 2016
4
Thank you all for your answers. Can somebody recommend me some
book or article about grounds and measurements? Thanks.
 
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