OLED diplay misalignment with Beaglebone Black

[a_power]

Dear all,
I have a Monochrome 1.3" 128x64 OLED display connected to a Beaglebone Black. I am working with the SPI bus and I am using the python examples furnished by Adafruit. It is working correctly with the shape.py example.
Now, I am trying to display on the full screen some "A" and then to switch to some "B" with a refresh rate of 1 s. In the picture "Good Screen view", you can see some "B" as I said.
However, from time to time, I see something strange in the display, such as a misalignment on the screen and I cannot understand what is going on (image "Bad Screen View").
I tried to take a look to the SPI signals, to the Reset and the DC signal on the oscilloscope and they seem to be good (even if it is not possible to trigger on an event giving a bad screen view)
Do you have any idea about what is happening?
Thanks a lot.

[a_power]

Dear all,
one more screenshot that can be useful.
As you can see, at a certain point, the "A" character is missing the horizontal line, which seems to be below; from that point, something strange happens.
Thanks for your help.
Regards.

[drewfustini]

It would be good to determine if the behavior is that results of your particular OLED display or a bug in the Python library.

Are you able to try controlling the display from an Arduino to verify it works reliably with Arduino library?

[a_power]

Dear DrewFustini,
thanks for your answer. Actually, I have not an Arduino with me to test it.
I tried with two different OLED displays and two different Beaglebone Blacks, obtaining the same results..
Thanks for your help.

[drewfustini]

What is the model number of the OLED?

If I have the same one, then I will try to reproduce the issue.

[a_power]

Dear DrewFustini,
thanks again for your reply. It looks like it is a problem of my hardware.
I have 10 ohm connected in series on the SPI Data and Clock signals going from the beaglebone black to the OLED screen.
They seem to be the reason of that behaviour. I removed them and it is correctly working (3 hours run, right now).
I do not understand why just 10 ohm can cause such a strange behaviour (do you have any idea)?
By the way, now it is working.
Thanks again for all your effort.
Regards

[culebra]

I have the strong suspicion, from the description you provided, that you have signal integrity (dirty signal) issues on your SPI connection.

There may be ground level differences between the BBB and the OLED, it can also be the cables you are using. Removing the series terminations (the name for the resistors you removed) improved the speed of change of the SPI signals and fix the problem in your case. Without an oscilloscope, is hard to tell how close are you still have it malfunctioning.

Suggestion:

1. Make sure that the Ground (Return or whatever name you want to give it), one of the cables that feed power to the OLED is thick. Make also certain that the same reference connection between the OLED and the BBB is thick (16 to 18 AWG) .

2. Your cables for PWR should NOT be close to the signal cables, unless they are part of a cable assembly.

3. There are other more complex things to address, but without knowing if you have control over them, it would be confusing in your case.

Hope you find this helpful.

culebra