Both 5" and 7" displays have 800x480 pixels, and when used on Windows, at least, will autodetect and set the resolution. On Raspberry Pi, you're better off forcing the HDMI resolution by using the following config.txt file (in /boot/config.txt) - you can edit it by popping the SD card into your computer, the config.txt file is in the root directory
Remember, the TFP401 driver does not have a video scaler! If you don't feed it exactly 800x480 pixels the image will not stretch/shrink to fit!
# uncomment if you get no picture on HDMI for a default "safe" mode
#hdmi_safe=1
# uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan
#disable_overscan=1
# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=16
#overscan_right=16
#overscan_top=16
#overscan_bottom=16
# uncomment to force a console size. By default it will be display's size minus
# overscan.
#framebuffer_width=1280
#framebuffer_height=720
# uncomment if hdmi display is not detected and composite is being output
hdmi_force_hotplug=1
# uncomment to force a specific HDMI mode (here we are forcing 800x480!)
hdmi_group=2
hdmi_mode=87
hdmi_cvt=800 480 60 6 0 0 0
hdmi_drive=1
max_usb_current=1
# uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
#hdmi_drive=2
# uncomment to increase signal to HDMI, if you have interference, blanking, or
# no display
#config_hdmi_boost=4
# uncomment for composite PAL
#sdtv_mode=2
#uncomment to overclock the arm. 700 MHz is the default.
#arm_freq=800
# for more options see http://elinux.org/RPi_config.txt
Also on Raspberry Pi — use the raspi-config tool to force audio to the 3.5mm jack (not HDMI or auto). Not all screens support audio over HDMI, and in this case it may actualy interfere with the display.
To let the Pi A+/B+ drive a display power over USB, first make sure you have a 2A power supply, with a good quality USB cable, a thin wire power cable is no good. Make sure its 24AWG or smaller, shorter USB cables are better too. Then add
max_usb_current=1
to /boot/config.txt
If you're getting wierd reboots, its likely the power supply and/or power USB cable is not good enough. A powered hub will also solve this problem
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