i have a 3.5" display that works perfectly but i don't know how to connect it to GPIO. I'm searching on internet, but i am finding nothing. how can i make it work? I need to force video output to GPIO. my display: https://www.amazon.it/Raspberry-Tablet-Pollici-Resolution-Interface/dp/B01CNLYL1C/ref=sr_1_5?s=electronics&ie=UTF8&qid=1505164535&sr=1-5&keywords=display+raspberry
You'll need to configure the screen. From here:
dowload the driver:
tar xvf LCD-show-*.tar.gz cd LCD-show/
Install the driver and it toggles the mode to LCD display: Note: Net work connection is required while installing driver to your Pi, or else the touch won't work properly.
chmod +x LCD35-show ./LCD35-show
Note: this LCD won't work after apt-get upgrade, in such cases, please edit the config.txt file in the SD card and remove this statement: dtoverlay=ads7846
After system rebooting, the RPi LCD is ready to use.
There is more info in the link provided. You should check it to see the options.