Adafruit HX8357D Display Backlight

Other Hardware Displays
1

The learning materials on Adafruit concerning the HX8357D display appear to indicate that when it is powered up, the backlight comes on. But also included is a pin labeled "Lite", and one reference in the Adafruit document that the backlight can be controlled by PWM. Nowhere did I find direct reference to use of the Lite pin, but I assume that if full on, it provides full light, and then if full off, it shuts off the light, with various levels of PWM providing analog control. But is that direct, or through using an RC circuit with PWM to get an analog voltage at the pin?

And does anybody know what power this consumes at full backlight?

Thanks in advance for any information.

2

I have the adafruit 3.5 tft w/ that controller and can confirm that when lite is grounded, the backlight turns off! Haven't tried PWM. Sorry...not much help.

3

I do not have this display. I have not read the documentation. I suggest that you do.

If you do not want to read, just try LITE with LOW or HIGH i.e. discover whether the backlight is active-low or active-high.

Then try digital PWM. It should work fine.

You should never put a RC on a switching transistor. This would extend the switching time and prevent it being in its on or off state. This could overheat the tiny transistor.

David.

4

David,

I don't ask questions until I have thoroughly research a subject. And I am perfectly capable of hooking it up and playing with it. So I'll give you some counter-advice. If you don't have anything to contribute, don't.

5

Would appreciate it if you'd post your findings here.

6

My apologies. I understood from your question that you wanted to connect an "analog" signal to Lite when Adafruit specifies digital PWM.

I gave a general "electrical" answer / explanation without reading your specific documentation.

I subsequently googled "Adafruit HX8357D" and found this pinout page

It says:

Lite - this is the PWM input for the backlight control. It is by default pulled high (backlight on) you can PWM at any frequency or pull down to turn the backlight off

So it is active-high. You will use regular non-inverted PWM as supplied by the (confusingly named) Arduino AnalogWrite() function as described in library reference

I repeat. A switching transistor should only be drived hard on or hard off. Any "smoothing" RC lowpass filter will be unwise.

The same will apply to a regular GPIO input or output pin. Do not drive a high capacitance e.g. > 100nF. It is less critical for a digital input. The CMOS will change state when it passes the input threshold. The output state will be either high or low.

David.