Hi all,

I'm trying to setup my oscilloscope to construct an eye diagram of some UART connection, however I am unsure as to how to trigger it properly without a clock. Is it possible to trigger my oscilloscope (HP54542A) so as to produce a diagram without a clock source? If so, how? I have tried playing with the pattern, state, delay triggers, etc, but if I'm honest, I am simply guessing.

I have managed to at least produce a portion of an eye diagram, see below.


Many thanks for your time,

Sparky

 

Try triggering at midpoint, and triggering on both edge polarities.

 

Hi Kubeek,

Thanks for your reply. Would you be able to give a bit more detail, please? If it's scope specific, don't worry, but, for example, which type of trigger should I be using?

Many thanks for your time.

 

I would use triggering on edge, and set it to trigger on both rising and falling edges. Set the trigger level in the middle between low and high logic level, and move the trace a bit to the left so that you can see the whole eye.

 

Many thanks for your reply again. However, I cannot seem to see a way of both rising and falling edge triggering on the HP54542A, so would it be fair to say that it is impossible to perform on this instrument?

 

This was the best I could figure out.

 

First you don´t want it to trigger automatically, but only at the trigger condition. Second I am not sure if it is truly triggering on both edges, can you somehow verify that?

Edit: yes, it looks like that the scope cannot do triggering on both edges.

 

Thanks for your time though.

 

I'm trying to setup my oscilloscope to construct an eye diagram of some UART connection, however I am unsure as to how to trigger it properly without a clock. Is it possible to trigger my oscilloscope (HP54542A) so as to produce a diagram without a clock source? If so, how?

No, it is simply not possible to get a meaningful eye diagram by triggering on the waveform you are trying to evaluate, regardless of the scope's triggering mode or threshold settings.

Think about it: the scope triggers on a specific voltage level, and the point in time when the signal crosses that threshold is always displayed at a specific horizontal position on the scope screen, no matter the actual time that has elapsed since the last threshold crossing. Your signal could actually be jittering like crazy, and you would never know it because the scope will neatly line up all the individual traces on top of one another at the threshold voltage crossing points.

Without access to a true UART bit clock signal to trigger on, your best approach will probably be to trigger on the signal at the transmitting UART's TxD pin, before the line driver chip. While that won't give you a complete "eye" diagram, it will at least give you some useable waveform quality information.

To get a real eye diagram, you'll have to disable the UART on the transmitting end and feed the line driver chip with a pseudorandom binary sequence (such as from a LFSR) clocked at the same rate as the UART's baud rate, and trigger your scope on the LFSR clock signal.