[Balloon] Balloon 3 feature of the day: switch-off and low-b…

I spent some time over the weekend adding a couple of features to the
Balloon 3 kernel. It can now switch itself off, and programs can detect
the state of the power supply. This is essential in battery-powered
applications.

The first is the ability to switch off the Balloon. This is implemented
so that the standard 'shutdown -p' or 'poweroff' commands Just Work. It
has a couple of caveats:
- it's not really 'off' but deep-sleep mode. The board can be woken up
again by a signal on GPIO0 or, I think, GPIO1, as well as the obvious
hardware reset.
- the audio codec doesn't get shut down properly so its analogue
entrails are still consuming power. On my test board, a 3v32 build, the
current consumption was 1.5mA. On a new 3v33 board it's 7mA. I'm not
sure what's going on there yet.

Poweroff required a kernel change (in balloon3.patch) and a minor tweak
to the CPLD, so that GPIO0 is held low rather than high by default.

The second feature is the ability for userland to see the state of the
power failure signals connected to the processor. There's now a kernel
module, balloon3_powerfail. It's not big and it's not clever, but it
creates a device /dev/powerfail which returns on each open and read a
hex number. Bit 0 is nPWR_OK and Bit 1 is BATT_FAULT.

nPWR_OK indicates whether the power supply rails are in regulation or
not. As the power supply drops, the 3.3V rail is the first to falter,
though the board carries on running down to well below 3V. 3.3V is
actually quite a convenient voltage to stop the discharge of Li-Ion
batteries.

BATT_FAULT goes high when the battery voltage drops to 3.15V. At this
point a LiIon cell really doesn't have much left.

I've used a combination of these two features in a battery-powered
application so that it can do an orderly shutdown, save state, and then
switch itself off as the battery runs down, without the need for any
external power management or battery monitoring.

The kernel changes are in 2.6.29.1 trunk. In the usual fashion, it's as
well to keep bootldr and cpld in step.

Chris
--
Chris Jones - chris@martin-jones.com
Martin-Jones Technology Ltd, makers of Solidlights
148 Catharine Street, Cambridge, CB1 3AR, UK
Phone +44 (0) 1223 655611 Fax +44 (0) 870 112 3908
http://www.solidlights.co.uk/