SSD1306_I2C poweron method

[belimawr]

I was using a OLED display with the SSD1306 diver and noticed that the SSD1306_I2C class does not implement the poweron method.

Looking at the init_display I found way to implement it that seems to make sense and worked very well for me.

[peterhinch]

Out of curiosity what physical problem does this fix? The existing driver works fine here.

[belimawr]

This is not a physical problem. Actually it is a feature. The I2C driver (SSD1306_I2C) does not implement the poweron method, so it is not possible to turn the display off the turn it on.

[kfricke]

That is only partially correct. The poweroff() method does switch off the display, but the current driver does initialize the display using the init_display() with full contrast set and resetting all other settings.

So the addition of this PR lets me assume that it would allow a user to physically save power by turning it off and reactivate it on the previous contrast level.
Using the init_display() method would likely let the display flicker a little in the worst case, because the contrast would need to be reduced again after reactivating the display.

One segment of the display seems to consume ~25uA at lowest contrast compared to 100uA on full contrast.

To really prove the usefulness one needs to...

1. ...verify that the method really saves reasonable amounts of power. Otherwise this only makes sense to me in a semi low power situation (as far as i do understand the datasheet, but i can not really grab the precise benefit or proove that it than only consumes ~10 uA).
2. ...check that the display does awake in the exact previous state. Else driver would better be enhanced by persisting the set contrast level in a member variable.
3. ...take into account that my displays of this type did never support reducing the contrast level at all.

[mcauser]

It's not just reactivating the display with previous contrast level. The previous state is persisted in RAM.

You can write "Hello" to the display, invert it, power it off, power it on and Hello is re-displayed inverted.

display.text('Hello',0,0,1)
display.invert(True)
display.write_cmd(0xae) #aka display.poweroff()
display.write_cmd(0xaf) #proposed display.poweron()

Without this PR, calling power on (pass) makes you think you have powered on the display, but when there is nothing displayed, you call init_display() which reinitialises everything and you lose your previous state.

The datasheet specifies 0xAE for display off (sleep mode) and 0xAF for display on (normal mode)

[kfricke]

There should be no breaking changes to existing code by this (atomic) PR. But instead it is implementing a missing feature.

[mcauser]

https://cdn-shop.adafruit.com/datasheets/SSD1306.pdf - Page 37

10.1.12 Set Display ON/OFF (AEh/AFh)
These single byte commands are used to turn the OLED panel display ON or OFF.

@dpgeorge this one is safe to merge.

[dpgeorge]

Thanks for confirming @mcauser , now merged in 7df4083

And then in ca2427c I updated the SSD1306_SPI driver to have the same behaviour: do a soft power-on when this poweron() method is called.