On Sat, 24 Oct 2015 09:02:20 -0700 (PDT),
arunbarnabasjohn@gmail.com declaimed the
following:
I have seen many Android TV stick devices running via USB or HDMI, but
these do not have battery backup, but instead rely on a TV for power. How
are these devices able to prevent shutdown problems ??
Hypothesis...
Probably by not using a live "file system" (with or without
journalling). Any configuration data may be written to straight flash
memory internal to the processor. The operational code is fixed in flash,
and only the stack may be in a limited amount of RAM... Not like the BB (or
RPi, Galileo, Edison) which load applications stored on external flash
memory using a file level protocol, and run them from RAM. They may also be
using a "supercap" charged by USB (don't know if HDMI carries DC power,
presume so if the device can run in an HDMI port) -- with a brown-out
detect circuit that does a clean shutdown once the capacitor drops below
some limit (or on loss of USB/HDMI power).
Or to rephrase somewhat -- if you were to dump the memory map of these
dongles, you might find something like
0 RAM (256K)
256K flash (512K)
768K memory mapped I/O registers (256K worth)
1M end-of-address space
where the applications in the flash are, say, position independent code (so
they will run from anywhere they were written -- one only needs to jump to
the entry point).
Most Android tablets are rigged such that it is difficult to actually
remove power from them (you can't open them to pull out the battery). The
"power" switch normally just blanks the display and hibernates the
processor (leaving an RTC running -- and that runs even if one DOES force a
clean power-off state by holding the switch down for some time)
And if one did manage to drop the power without a shutdown, the
power-up sequence probably masks that the unit is running a full validity
check of the flash image before going out to "storage" and checking it.
The application I am planning to run will not use the SDcard or any other
backup devices, the application will reside in the SDCard though , but
hopefully would run entirely from the RAM. This way I hope to prevent any
problems due to improper shutdown...
If any paging takes place, you've got updates to the file system. A
clean shutdown ensures the file system journaling has been flushed.
Rigging a capacitor and brown-out detection circuit to trigger a
shutdown might be the safest solution.