Sunday, May 3, 2015
Adafruit Raspberry Pi displays
There are two interesting displays for the Raspberry pi that complement the HDMI and Composite video outputs. Both are available with or without touch screen capability.
One is a 2.8" or 3.5" screen for the pi that is supported by the original 26 pin connector GPIO. Only some portion of the GPIO is used, and the support for both the touch screen and display are all thru the connector.
Another one is a Hat which only works with the 40 pin GPIO on the Pi B+ or the Pi 2 Model B. Almost all of the GPIO is used by the hat, and no pins remain to support the touch screen, so it is brought out via USB port as a motion event device to the Raspian system.
The latter's support compiled from a series of articles from the Raspbian / Debian system and does not apparently have versions for other OS's. I didn't track down anything to do with the other OS's anyway for either.
The Hat version is product number 2453, and a 7" display was chosen to have 800 x 480 support. a reasonably sized display results that can run a lot of graphical displays.
The smaller version has 320 x 240, and is quite good, but most gui dialogs will only show a very small portion on the screen.
Additionally the support for the smaller piggyback displays is via loadable modules, and they are not initialized soon enough in the scheme of things in the kernel to have much of the boot dialog. There is a point in the normal kernel bootup where the kernel switches from the initial cached console dmesg stream to the running kernel device.
Kernel traffic comes out to the small screen versions after that point, and an X capable frame buffer results from the modules and mods to the system.
The Hat version is interfaced early enough to have the full boot, including the kernel graphic raspberry logo on displayed. It also has a frame buffer added and supported.
Support of X on bootup is much more simple with the Hat version than with the smaller displays. The latter require a bit of familiarity with frame buffers and following hints in the support files to get the X display to be the default display. Doable, but was messy.
Drivers for display:
Pinout for GPIO (worksheet)
Thread about B+ addon:
480 x 320 3.5" display:
INstallation of blob over Raspian
Note that the hat, and display are integrated and only a single part is necessary.