Electronics > Microcontrollers

Single-ended serial transmission question.


Randall W. Lott:
On my recent project, which contains a microcontroller and a peripheral UART device, I am getting transmissions that appear to have a fairly substantial slope.  The appearance of short "1's" look triangular.  Is this because of the 50MHz oscilloscope bandwidth acting as a low pass filter, or is it a design issue?

The baudrate is slow; at 9600.  I don't have access to a logic analyzer.

I assumed that slow serial streams could be viewed in decent detail with a 50MHz oscilloscope.

Thank you.

Apparently there is nothing wrong with the scope. You should have a problem in the circuit.
Post the schematic if you can.

Randall W. Lott:
Actually, it's a complete module that transmits a stream.  Insufficient power supply filtering?  I'll hook it up to a lab supply and see if the data is slewed.  I used a capacitor that is larger than the recommended value and used decoupling capacitors on every power pin on my design.  I designed my power section to provide more current than worst case.

You can see each sample the scope took of the signal.
It looks like you zoomed in on scope memory a little far, rather than a problem with the signal itself.

Try using the trigger/holdoff settings to capture the signal at a faster timebase, rather than zooming in on a slow timebase capture.

I think you will find the signal is actually nice an square.


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