[GTA02] clock reset on battery removal?

[Werner Almesberger]

Paul Wise wrote:
> Yet another hardware design issue I guess, I'm quite surprised there
> isn't a small battery to power the clock like in PCs.

There is. It's called the "backup battery". It's the small "button"
next to the front speaker. Strangely, this battery has a very
pronounced death wish. I.e., in a recent survey, something like 33%
reported that their backup battery can't hold a charge for any
useful amount of time.

(I don't remember who conducted the survey. Joerg told me about the
results.)

It's not clear what causes the backup battery to perform so poorly.
It could be a quality issue, it could be a manufacturing issue (*),
it could be incorrect use in the GTA02 design, it could be a usage
pattern of the user that subjects the battery to unexpected stress,
etc.

(*) We had backup batteries die like flies at some point in time,
    GTA01 or early GTA02 prototypes I think, because they were not
    compatible with the soldering profile.  However, the battery
    was replaced with a sturdier one, so this is supposedly no
    longer a problem.

In general, I think it would be good to avoid assuming that the
backup battery works. There are many potential time sources around,
GPS being only one of them, so I think a distribution that detects
a loss of RTC could cushion the effect in many cases.

There are also time sources that are exact but have an unknown
offset, such as the time provided by some GSM providers. When the
system has a know to be good time, it could identify and record the
time offset, and use this information later to derive the correct
time from this kind of source. Just throwing ideas in the general
direction of the middleware folks :-)

- Werner