UART Baud and Bit rate Measurement With a scope.

[jay_dee]

HI, I'm battling UART and my trusted method to check Baud is to measure the width of a bit using a scope.
When sending 0x55, I have always used the Cursors to measure a single high or low pulse width.

I'm getting some subtle timing errors so I thought I would check my basic method with google, then ChatGPT (sorry, I know!)
but ChatGPT gave this method that measures the PERIOD of the wave form...surely this is wrong?

ChatGPT gives...

3. Correct Measurement Technique
Send a repeating character (0x55 ideal), because it produces a clean alternating bit pattern (10101010).
Set the oscilloscope trigger on a falling or rising edge.
Zoom in until you see several adjacent bit transitions.
Use cursor Δt or automated period measurement between two similar edges (e.g., rising → next rising).
Convert the measured Δt into baud = 1/(Δt).


Below is the Methods I beleive to be the propper measurement technique. (image robbed from internet!)

bit.jpg (84.02 KiB) Viewed 67 times

thoughts? J.

[LeighM]

Yep, AI is wrong, as usual
Depends what you asked though.
Your screen shot method is correct for UART baud.

[jay_dee]

Thanks for the clarification Leigh! exactly what I needed.

Fair Point regarding the question, everything is down to interpretation.
I asked "when measuring UART baud rate with an oscilloscope do you measure the period or pulse width"
hey ho. J.

[medelec35]

Hi J.
This is what I have stated in the past.

If you have a scope or a logic analyser Just send an upper-case U and measure either one bit mark or space duration, then baud rate = 1/mark or space duration.