Electronics > Microcontrollers

Single-ended serial transmission question.

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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.

johnmx:
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.

Psi:
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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