Electronics > Projects, Designs, and Technical Stuff
Clock Battery Monitor Circuit
Peabody:
I understand, and that may be the case, but if "isolation" disconnects the battery ground from the main ground, you won't get any current flow even if the ADC pin was a direct short to its ground. For current to flow, there has to be a path back to the negative terminal of the coin battery from the system ground. If that's switched off, no current will flow. The standard DW01 lithium protection circuit does exactly that, so it's not impossible that they could have done the same for this chip.
bitogre:
--- Quote from: Peabody on July 19, 2020, 06:44:38 pm ---I understand, and that may be the case, but if "isolation" disconnects the battery ground from the main ground, you won't get any current flow even if the ADC pin was a direct short to its ground. For current to flow, there has to be a path back to the negative terminal of the coin battery from the system ground. If that's switched off, no current will flow. The standard DW01 lithium protection circuit does exactly that, so it's not impossible that they could have done the same for this chip.
--- End quote ---
Ok, so a standard DW01 lithium protection circuit is another option but only if using an external RTC. Interesting. Will have to investigate that further.
Peabody:
--- Quote from: bitogre on July 19, 2020, 06:54:24 pm ---
--- Quote from: Peabody on July 19, 2020, 06:44:38 pm ---I understand, and that may be the case, but if "isolation" disconnects the battery ground from the main ground, you won't get any current flow even if the ADC pin was a direct short to its ground. For current to flow, there has to be a path back to the negative terminal of the coin battery from the system ground. If that's switched off, no current will flow. The standard DW01 lithium protection circuit does exactly that, so it's not impossible that they could have done the same for this chip.
--- End quote ---
Ok, so a standard DW01 lithium protection circuit is another option but only if using an external RTC. Interesting. Will have to investigate that further.
--- End quote ---
I'm just saying that's something they could have done to make the internal RTC truly isolated. I just want you to do the test I described above to find out.
For an external RTC module, it might be as simple as an N-channel mosfet switching the Teensy ground to the external module ground, with the gate controlled by a Teensy GPIO pin. But you would have the same issue as before, but in the opposite direction: when the main power is turned off, does that pin go to ground, or does the positive voltage applied to the ADC pin keep it from doing that? And of course there are the I2C lines between the module and the Teensy. I don't know what happens to them.
Peabody:
Attached is a circuit that connects an external RTC module's ground to the Arduino ground only when the Arduino is powered up. I had hoped it could be done with one transistor, but I don't see way to do that. I breadboarded this, and I see no current flow out of the coin B+. However, when the Arduino is powered up, there is current flow through the base resistor of the PNP and the gate resistor of the mosfet. So it might be best to connect the emitter and base of the PNP to GPIO ports so they can be turned on only when you need to measure the voltage or communicate with the RTC.
bitogre:
--- Quote from: Ian.M on July 19, 2020, 04:40:21 pm ---An OPAMP's input impedance when unpowered depends on its exact topology and input protection network and is rarely specified.
--- End quote ---
I am thinking of using a LM741 Op Amp (https://www.ti.com/lit/ds/symlink/lm741.pdf). My testing of it seems to have a really high impedance with no power (which make sense given the specified functional diagram) and a reasonable Input Bias Current. I can give it 5V power to ensure I have enough head room on the output when powered.
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