PiScreen 3.5" TFT only shows white

Question

I am attempting to set up a Zynthian Live Sampler (https://www.zynthian.org) which uses the Ozzmaker PiScreen 3.5" (http://ozzmaker.com/product/piscreen-3-5-tft-with-touch-for-the-raspberry-pi/) as it's display.

However, when I power on the Zynthian device, the screen turns on to show only white whilst all other functions work as expected. I have tried placing the screen on a different Pi with the required drivers installed to receive the same result. This only occurs when the male pins of the screen are used, and the female header that would normally be used to connect the Pi directly operate fine.

The cable used to connect the screen is rather lengthy at this point in the build, could this be an issue? I have ensured all pins of the cable are connected properly.

Thanks.

Answer 1

It turns out the cable quality was just very low, and this distributed over a relatively long piece of cable caused issues.

As quoted from @jofemodo on the Zynthian discourse:

About capacitance in flat cables, a typical (good!) value is about 45pF/m. Resistance is about 100Ohm/m. So, for the official zynthian ribbon bus, we have:

C = 6.75 pF R = 15 Ohm

Using this Low Pass Filter calculator:

http://sim.okawa-denshi.jp/en/CRtool.php

We have the cut frecuency at about 1GHz, quite far from the 32MHz used by the PiScreen SPI connection. Not problem with it.

Lets suposse we have a bad-quality cable (100pF/m), and quite long (40cm):

C = 40 pF R = 40 Ohm

For that case, the cut freq is about 100MHz, what is too close to the 32MHz of the SPI bus freq. Not good and probably it wont work.

Thanks for the help.

Answer 2

White screen means (almost) a bad screen resolution.

Use a regular screen, go in the "preferences" and try to select the same resolution the 3.5" TFT has...