Battery Backup For on Board Overo Board RTC (twl4030)

I have been googled a lot for the external rtc project. Almost people are discussing about how to make the external rtc working. Seldom to see people mention about the on board rtc usage.

Have a look on gumstix website:
7) The Overo series does not support battery charging.

the battery charging was disabled for safety concern.

However, take a look at this new implementation, line 470:;a=blob;f=drivers/rtc/rtc-twl4030.c;h=72f7863376c8eb61ffbe3c5e51bc0e525744aa09;hb=83d22678ac1d8126b41f6880f57e2421d0c11391

470         /* enable backup battery charging */
471         /* use a conservative 25uA @ 3.1V */
472         ret = twl4030_i2c_write_u8(TWL4030_MODULE_PM_RECEIVER,
473                 BIT_PM_RECEIVER_BB_CFG_BBCHEN |
474                 BIT_PM_RECEIVER_BB_CFG_BBSEL_3V1 |
475                 BIT_PM_RECEIVER_BB_CFG_BBISEL_25UA,
476                 REG_BB_CFG)

Measure the voltage at pin12 (Vbackup) while the gumstix is on. There is 3.13V on it.

It seems like the battery backup charging truly has been enabled!

try it anyway...

bought a coin cell battery, connect to Vbackup and Gnd,

root@overo:~# hwclock
Thu Oct  7 06:39:02 2010  0.000000 seconds
root@overo:~# date
Thu Oct  7 06:39:17 UTC 2010
root@overo:~# date 12090857
Thu Dec  9 08:57:00 UTC 2010
root@overo:~# hwclock -w
root@overo:~# hwclock
Thu Dec  9 08:57:14 2010  0.000000 seconds

turn off gumstix, wait for10 minutes, start gumstix,

during the boot process:
fbcvt: 1024x768@60: CVT Name - .786M3-R
Console: switching to colour frame buffer device 60x34
regulator_init_complete: incomplete constraints, leaving VDAC on
twl_rtc twl_rtc: setting system clock to 2010-12-09 09:09:40 UTC (1291885780)
Waiting for root device /dev/mmcblk0p2...
mmc0: host does not support reading read-only switch. assuming write-enable.
mmc0: new high speed SD card at address b368
mmcblk0: mmc0:b368 UD    1.90 GiB

root@overo:~# date
Thu Dec  9 09:10:25 UTC 2010
root@overo:~# hwclock
Thu Dec  9 09:10:38 2010  0.000000 seconds

Hmm, how convenient! Great, throw my external rtc away ~

Lithium Vanadium Pentoxide battery, Farnell-1514218

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