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Battery/Supercap backed RTC on TM4C1294

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I've recently started using the real time clock on the TM4C1294.  I have the clock working, keeping time, and set with NTP.  I want to try and make it a battery backed RTC though.


I'm currently wasting time in an airport, so I can't test.  From the schematic, it looks like if I remove R39, and connect a battery to TP11 and gnd - I should be able to keep the clock going while the board is powered down.  


Has anyone tried this, or done it with a custom board?



Edit - Oct 10th.  Finally got a chance to try it, and it does work!   I put 3.3v from the bench power supply on TP11, with R39 removed and the RTC stayed running and on time while the main board was powered down.  Next step is to try a small supercap in place of the bench supply and see how long I can keep the RTC running off that.

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Small update:


I charged a 0.1F supercap to approx 2.9v, and disconnected the power from my board.  Since last night (8 hours) it's down to 2.03v.  Clock is still in sync (within reason for a crystal oscillator).  Datasheet says 1.8v is the low point for vBAT.



I just restarted my test, with the supercap fully charged to 3.3v.  Let's see how long it will run.


Edit - 12 hours later.  Supercap is at about 1.82v.  So not too bad!

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