revisiting ECN0040 (backup battery)

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revisiting ECN0040 (backup battery)

Werner Almesberger
The backup battery situation merits a second look. In addition to
the material already in ECN0040, we have the recent survey that
quite strongly suggests that the backup battery simply is no good,
and that the problems may not just be caused by regular aging (as
I assumed in ECN0040.)

See also my comment here:
http://lists.openmoko.org/pipermail/community/2010-April/061200.html

So I think the case for removing the backup battery is getting
stronger. I also very much like the idea of having a PMU reset
state that doesn't depend on what the system did before the
reset.

We have a capacitor to keep up the modem's RTC, so there's at
least one RTC in the system that reliably survives a complete loss
of power for a short while.

Does anyone want to save the backup battery ?

If not, what do we put in its stead ? Choices include nothing or a
resistor. The backup battery charger can be turned off, so no
power would be lost in the resistor under normal conditions.

- Werner

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Re: revisiting ECN0040 (backup battery)

Patryk Benderz
[cut]
> Does anyone want to save the backup battery ?
I am not sure if that applies to this problem, but I would like to have
some backup rechargeable battery on board, to keep date ant time set
between swapping normal battery. Its just a quick answer, cause i had
just few moments to read mails. I will think over your other questions
in this post and reply later if find out something useful.

--
Patryk "LeadMan" Benderz
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Re: revisiting ECN0040 (backup battery)

Werner Almesberger
Patryk Benderz wrote:
> I am not sure if that applies to this problem, but I would like to have
> some backup rechargeable battery on board, to keep date ant time set
> between swapping normal battery.

gta02-core has a large capacitor to buffer the RTC in the GSM modem.
This RTC has a lower power consumption than the one in the PMU, so a
capacitor can supply it for a little while.

We can use a tantalum capacitor, which doesn't age perceptibly.

The 220 uF capacitor in gta02-core should preserve time for about
two minutes, provided that the GSM modem is powered when a battery
is present.

Thus, software could do the following:

- when the PMU's RTC has a valid time, continuously synchronize the
  GSM modem's RTC with it (if the modem is unavailable, e.g.,
  powered down, do nothing)

- if the PMU's RTC does not have a valid time (e.g., the year is 00)
  and the GSM modem is operational, set the PMU's RTC from the GSM
  modem.

- in all other cases, it could try to obtain a valid time from
  external sources, such a NTP, GPS, time from the GSM provider,
  or user input.

Time would always be lost if power is removed for more than about
two minutes or if power to the GSM modem is cut for more than two
minutes before battery removal.

- Werner

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