Wihtout power supply,how many times can keep on the internal clocking with the Super CAP ( VDD_RTC of the Jetsom TX2 4G)

Hello. I am having problems with the VDD_RTC PIN,I want to konw how many times can keep on the internal clocking with the Super CAP ( VDD_RTC of the Jetsom TX2 4G).then I can know the hold time of the core-board

Hi, the working current of RTC part is about 2uA, you can calculate the working time accordingly based on capacitance of super cap.

Hi, there.

Let us start from

I(t) = C x dV(t) / dt.

Since the working current of RTC is about 2uA, as Trumany pointed out, I(t) can be simplified as I = 2uA, a constant independent of time. We further assume V(t), when discharging, is dropping linearly with time, so dV(t) / dt = delta V / delta t. From datasheet, C72 is a 80000uF super cap, and VDD_RTC won’t work when the input voltage drops below 1.65V.

Now, putting everything together we have

delta t = C x delta V / I
= 80000uF x (3.3V - 1.65V) / 2uA
= 66000s
= 18.33h

Math tells me TX2, when disconnected from power, could maintain its internal clock for 18.33 hours. But my personal experience is that the system normally cannot survive one night. If I’ve left mistakes along the line of my thought, please feel free to let me know.

Additionally, what is the best practice (with minimum changes made to the TX2 carrier board) to extend the working time (to one week for example)?

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The full charged backup cap voltage is typical 2.5V not 3.3V.

delta t = C x delta V / I
= 80000uF x (2.5V - 1.65V) / 2uA
= 34000s
= 9.44h

OK, that’s more like it.
Then, again, would you please kindly recommend the best practice (with minimum changes made to the TX2 carrier board) to extend the working time (to one week for example)?

Let me add some more information as of what I’ve further tried.

I am lucky enough to own two sets of TX2. The time and date of one set, say TX2 A, have been manually set up-to-date, and C72 fully charged. Then I use the other set, say TX2 B, to charge C72 before disconnecting TX2 A from power. Specifically, the 3.3V pin (e.g., pin 2 of J26, or pin 1 or pin 17 of J21) of TX2 B is wired to the positive terminal of C72 on TX2 A, and both A and B are common grounded. Think of adding a source of constant 3.3 voltage parallel to the super CAP C72.

What beats me is that even with this configuration, RTC of TX2 A still winds back to 2015 after several hours.

You just need to connect two C72 in parallel so as to compare the time to single C72.