[gta02-core] 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