Hey @pierred and @crami25 - no offense, but it looks like you are hijacking this thread to discuss a different topic, i.e. how to wake from sleep. The original topic of this thread is "how to enter deep sleep while keeping the LCD on". This is still not solved and I doubt that it will ever be.

Hey @mati. Any success on this?
Just to be clear, the issue is not that I need to cut the LCD in deep sleep, but just the opposite - to keep the LCD ON while the M5Stack is in deep sleep. Have you managed to test this?

Hey @ajb2k3, thanks for your input, but I'm afraid bigger/external battery doesn't answer OP's question and neither the issue I referenced. An e-paper display will certainly render the M5stack a lot more energy efficient while preserving the same product size (while external battery will just make the product bigger). And yes, e-papers of this size do exist, though are probably more expensive than the OLED.

@Mati if your M5stick is indeed in deep sleep while the display is on, then the issue I referenced earlier affects only the M5stack. I don't see why this issue is not fixed in this product...

I would really like to have an M5stack with e-paper display! I do have one of the use cases you mentioned - bitcoin price ticker, but I'm not using M5Stack due to several limitations, the biggest of which is that the display cannot stay ON while device is sleeping.

If anyone develops an M5stack with e-paper display (and eliminates the above limitation), I will gladly buy large quantities of this device!

The M5Stack can display JPG images (compressed, lossy) and BMP images (uncompressed). BMP is a rather archaic format, so I was wondering if the M5Stack can display the more modern PNG format (compressed, lossless)?

Alternatively, how can I use the drawBitmap function starting from a PNG image? I need the process to be automated, so using 3rd party conversion apps is not an option. The conversion (PNG -> bitmap data that the M5Stack can display) can be performed either on the M5Stack or on my external server using Node.js. Raise your hand if you know how it can be done :)

I agree with @ajb2k3 that M5Stack is a young project, so that indeed explains the lack of such queries. However, I don't think my use case is unusual - I can think of a lot of applications that would benefit from sleeping while the display is on, such as: a clock showing hours and minutes only (so waking up only once a minute); sensor monitoring application (e.g. temperature, humidity, etc.) that constantly displays the latest values and wakes up once each X minutes to update measurements; displaying the number of twitter followers, likes, page visits, etc. In all these use cases it is useful to have the CPU sleep most of the time while the display remains on.

In any case, it would be great if someone can present a working solution. It is probably just a few lines of code, but I just can't seem to get the right ones.

I can't believe this problem has no solution: keeping the display of M5stack ON during light or deep sleep. It should be trivial and yet no one on this forum can present a working solution. This seems to be an impossible task!

Thank you @watson and @ajb2k3. I did look into the documentation before asking this question, but it wasn't very clear to me how to put this into practice. That is why my question asked for a working example. I will still appreciate if someone could post that.

@world101, I just tested that commenting out Speaker.begin(); in M5Stack.cpp seems to get rid of the short glitch at startup. That did the trick for me, let me know if that works for you too.

Of course that applies only to projects that don't need the speaker at all. :)

Can anyone provide a working C++ example how to hold a digital output pin HIGH during ESP32 deep sleep? Thanks!