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To write a driver for a display I set up up the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Whilst developing the display driver I would regularly plug in the USB port to both upload programs and run the programs to check the signals with a scope etc. I would only sometimes switch on the external PSU that powered the display board.

After sometime I managed to get the display driver to work, but shortly after that the display image became heavily distorted. Looking at the spec for the ILI9486 I noticed that you shouldn't send logic signals whilst the display is not powered. I guessed that my development practice of only powering the Pico from the USB and NOT powering the ILI9488 was the cause.

In the meantime, I've decided that I need a bigger display that uses a RA8875 instead of the ILI9486. This means that I need to develop a new driver, but do not want the same problem with powering up as before.

Is there some - simple - way to improve the circuit so that either:

  • The display is guaranteed to be powered up before the Pico or
  • Signals to the display are inhibited if the display is powered down?
Andrew Doble
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1 Answers1

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Sure. There are many simple ways.

For instance, you could read with a GPIO pin if the display supply is on or off. If it is off, then don't initialize and don't try to communicate with the display.

Another way is to just have one single power supply for both devices, so you don't need to do any tricks.

Justme
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  • Thanks for the answer. Using a GPIO pin to measure the voltage strikes me as an elegant solution that uses no extra components. I guess I should set the pins to high Z before checking the power voltage and then I’m safe. – Andrew Doble Nov 06 '22 at 16:33
  • Depens how the display works. If some of the pins has a pull-up, then that can be used. But in general, having different parts of the circuit powered by different supplies must be given some thought so that currents do not take paths they should not and damage either the MCU or the display. – Justme Nov 06 '22 at 17:00