Positron BASIC Community

These displays use a 3.3V regulator which will simply saturate if you feed it with 3.3V and generally they will run down to 3.0V without a problem.  While they will run happily on a 5V supply due to the regulator there is a problem if the micro feeding it is running off 5V.  The drive from the micro to the display will tend to overdrive things but the real issue is the 3.3V ACK signal from display back to a 5V micro.  If using the micro's hardware SSP, the display's ACK signal falls just short of the IIC standard and the display may have broken characters or characters that haven't cleared.  The displays do have pull-up resistors added but they are fairly high in value (10k) but I have never had a problem with them on a low capacitance bus.  You could try adding a pull-up from the SDA line to 5V line (or 4.2V in your case).  This generally overcomes the ACK signal problem at the risk of wishboning the display silicon.
I have also found the regulator on many of these OLED modules rather noisy and on some brands you will see that C2 is missing.  This is the capacitor across the output of the regulator (stability pole).  Adding a 1uF MLCC will fix this.  The regulator devices are multi-sourced and it's possible one brand proved totally stable without the capacitor.
The useful range of a single LI Ion cell like a 18650 or 21700 is 4.2V down to 3.2V after which it tends to fall off rather steeply.  The display is fine over this range but the logic levels can be an issue.

Cheers,
David

Also note the pin out of the displays changes with brand and even between the 0.96" and the 1.3" sizes.  My photo shows Vdd and Gnd reversed when compared to the earlier front photo.

I think we can install 2.5 V regulators in the indicator and use a PIC12LF1840, and power it from 2.5 V as well. I think it should work, and the circuit should operate within a voltage range of 4.2 V to 2.7 V.

Thanks.

Hi,
I think the logic will work down to that voltage but it is below the minimum operating voltage for the on-chip capacitive charge pump.  Run some tests first before committing to anything and make sure the display starts up at the lower voltage and not just runs.
I wouldn't recommend operating the Li Ion cell down to 2.5V.  Yes, the manufacturer rates them down to there but they're not worried about cycle life and do that to maximise their rated capacity.  At modest C loading the voltage falls away fast below 3.2V.  Check out the attached graph of a good quality Samsung cell.  Capacity remaining below 3.2V is somewhat less than 10% and the area under the graph is energy which is even less again.
Just use the on-board 3.3V regulator as shipped and let it saturate down to whatever voltage you want to stop at.  So you could run the micro off its own 3.3V regulator or, if current drain is modest, I would try running it off the display's 3.3V regulator.