It is definitely not code related. Two reasons- the garbage is changing every time loop() runs, however there is nothing in loop() that alters the display.
Also it only happens in a sweet spot where there is exactly enough memory used up so that something goes wrong, but not enough used to cause the allocation to fail. Literally adding an additional unused global string variable filled with random characters will cause the garbage pixels to occur, and then adding a second String variable will cause a failed allocation.
For example I added these three variables:
If I run the sketch with only eatMemory1 uncommented, it works fine.
If I run it with only eatMemory2 uncommented, it displays the garbage pixels.
If I run it with only the third uncommented, the allocation fails.
I'm 99% sure it is a bug in the Adafruit library code that miscalculates the memory necessary to allocate the display. Which is why it only happens right in that sweet spot before failure.
Yes of course, but I'm not worried about fixing the problem, but rather why it is happening in such a specific way. The sketch only has display.print() commands in it, so it is not responsible for showing random garbage pixels in the lower right corner, that change every time loop() completes, despite there being no controlling code in loop.
Sorry, I should have worded my question differently. I was hoping someone familiar with the Adafruit libraries or the ssd1306's would recognize this garbage pattern.
I didn't want to burden someone with having to read through a thousand lines of code and/or replicate my circuit with identical hardware to reproduce the error.
Go on. You are well aware of the Adafruit_SSD1306 memory use. It allocates 1024 bytes of SRAM for a 128x64 buffer at runtime.
So your "Works great" sketch with 822 bytes (40%) at compile time turns into 1846 bytes (90%) at runtime.
This leaves only 202 bytes for stack, auto variables, dynamic memory.
Which is probably ok for modest programming with C strings i.e. char arrays.
However you appear to use C++ String which makes things very risky.
The normal advice is to put anonymous data into Flash memory. e.g. oled.println(F("unchangeable message"));
Try to get your compile-time SRAM use below 614 bytes (30%). i.e. runtime < 80%.
Oh, you wanted to know why the garbage is at the bottom right hand corner of the screen. This is where the stack has encroached onto the 128x64 buffer.
So the OLED is getting random bytes overwritten in the end of its buffer, and since the buffer starts at (0,0) for the top left and ends at (128,64) for the lower right, this is why it is happening in the lower right corner only, and not all over the screen?
So I would have assumed that the display.begin() function would return "SSD1306 allocation failed" if there wasn't enough memory. However it seems that is not always the case.
Is this in fact a bug in the library, that is happening because the ssd1306 is failing to allocate enough ram for itself, or is it simply an unwritten rule of arduino that stuff like this will happen when you approach the memory limit, and this is why you are telling me the <80% advice?
(I had assumed that since all of my memory allocation in the sketch is static (no use of malloc), the compiler would have given me the