I'm using a KMR-1.8" display which uses a ST7735 driver and it is working ok with the Adafruit_ST7735 library. The problem is the display is very dim once the library has initialized.
If I reset the board, the last display buffer is still visible and gets about 3 times brighter until the driver initializes the ST7735 again. If I hold the reset down, the last display in the buffer remains on the screen 3 times brighter for as long as I hold down the button.
The LED brightness pin is tied directly to the 3.3v output of a voltage regulator.
My conclusion is that there must be something the driver sets that effects the brightness. I don't see anything that looks like a brightness setting in the spec. sheet or in the driver source code.
Update: I've systematically disabled all of the driver settings except the
bare minimum and haven't been able to get the display to brighten up. I've even added commands to adjust the Gamma curve and it had minimal affect.
I've also breadboarded the display using a test rig and the display actually
worked without issue on the breadboard, maintaining it brightness. This used
the exact same register settings as the PCB version which dims.
Best Answer
There is no setting on the ST7735 that allows you to control the brightness.
You must PWM the LED backlight on your TFT display in order to control the brightness.
See the ST7735 datasheet here: https://www.displayfuture.com/Display/datasheet/controller/ST7735.pdf