[gta04] Meeting minutes for GTA04 discussion 20080411

Werner Almesberger werner at openmoko.org
Mon Apr 14 17:44:31 CEST 2008

Andy Green wrote:
> ~ - If we see any user interaction with the phone, ie, touchscreen,
> buttons, we always power the LCM backlight and S3C6400 ready to respond
> to what it is he wanted to do

Hmm, I'd leave the decision whether to power on the backlight or not
to the CPU. Before the CPU is up, there's nothing useful to display
anyway, and the CPU (application, really) may very well decide not to
bring up the LCM.

> Stuff that goes through the CPU has dead-time when the CPU is down, and
> loads the CPU when it is up.  For example, currently the motion sensors
> generate 200 or 800 interrupts a second on the CPU all the time neod is
> up, this has a power dimension.

That's in fact the perfect example for something the CPU can't do
usefully without expending great amounts of power (in relation to what
else is going on in the system), but something that's easy to do for
the MPU.

> We can still meet the gsm polling if we schedule wakeups from deep sleep
> externally or via RTC to do the polling, we still spend the vast bulk of
> time during the call with the CPU down.

I suppose we could just clock it down if we're sure there's nothing else
to do. That might bring CPU power consumption within a small enough
fraction of the modem's power consumption. (Which should be pretty high
during a call.)

At least in GTA01/02, the issue is also that the CPU has to receive
things on the UART, so almost no dead time is allowed. This may be
similar also in GTA04, just with protocols that aren't quite as easy to
handle as UART (which the MPU could do just fine, of course).

As I said, I think the MPU should try to do as little as possible, but
do it well, and keep plenty of room for future demands, since they will
certainly come. The more we can let the CPU handle, the better, since
this reduces the number of Openmoko-only quirks in our design.

> Let's identify what that other chipset/component does and costs then we
> can compare it.  I don't have shares in TI :-) but I know MSP430 is very
> low power and Milosch for example had success with it.

Also, it has JTAG, which is quite exceptional in the small MPU segment.

By the way, do you already have an idea which of the many MSP430 chips
and/or sub-families would suit our needs best ? I remember that Milosch
mentioned that some better supported Linux tools than others, so this
would be something to consider. It would be kind of sad of one had to
use Wine or Windows just to make some new firmware ...

> Right.  We perfer not to have the NAND at all and use a small 256KByte
> NOR to hold the bootloader.

For GTA02, we looked for the cheapest/smallest/lowest-power NOR we
could find, and ended up with 2MB. If we want to put a bit of a recovery
system (kboot-based) into the on-board Flash as well, so that people can
restore the SD card even if they only have legacy-free equipment without
SD card slots, 2MB of space would be about rock bottom. I'd feel more
comfortable with 4MB there.

- Werner

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