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Clock Battery Monitor Circuit

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bitogre:

--- Quote from: Peabody on July 19, 2020, 05:45:32 pm ---I found this in one of the Teensy datasheets:

"20.5 Reset and Initialization of SNVSSNVS is implemented in two sections (HP and LP) that both must be initialized by software following POR. If the SNVS_LP is powered by an uninterrupted power source that is separate from main SoC power, then SNVS can operate in either of two modes, depending upon whether the main SoC power is on or off. During main SoC power-down SNVS_HP is powered-down, but SNVS_LP is powered from the backup power supply and is electrically isolated from the rest of the chip. In this mode SNVS_LP keeps its registers' values and monitors the SNVS_LP tamper detection inputs, but the LP registers cannot be read or written. During main SoC power-up the isolation of SNVS_LP is disabled and both SNVS_HP and SNVS_LP are powered from the main SoC power."

I don't know what "electrically isolated" means here, but have you actually tested whether the ADC input sinks current from the coin battery when the main power is down?

--- End quote ---

I also do not know what "electrically isolated" means in that paragraph.  But, Yes, I put a multi-meter from ground to various ADC pins while the Teensy had no power (not even VBat for the RTC) and my multi-meter indicated about 1500 ohms from the ADC pin to ground.  Maybe it is not linear (not a pure resistor) and could change with different voltages but I think that does clearly show the input would sink power from the battery.

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 ---

Good point.  Will have to test once I get an op amp to try for this purpose.  Also, I could ensure a high impedance by adding a 100k ohm or maybe larger resistor between the op amp input and the battery.  That would ensure at least that much input impedance even if op amp input impedance dropped to 0 when it looses power.

Peabody:

--- Quote from: bitogre on July 19, 2020, 05:48:01 pm ---
Yes, the RTC shares a common ground.  Even if I went to an external RTC like the DS1307 or DS3231, it would have a common ground between the battery and the data lines.  So, I do not see how internal vs external RTC makes a difference on this problem.

--- End quote ---

Well, the difference is that you could just disconnect the separate RTC module when you go into storage, and connect it again when using the RV.  This could be entire module or just the ADC connection or the ground connection.  But you can do the same thing using the built-in RTC by just switching off the connection from B+ to the ADC pin.  Of course I understand that a switch is not as elegant as making it work automatically.

Peabody:

--- Quote from: bitogre on July 19, 2020, 05:54:32 pm ---
I also do not know what "electrically isolated" means in that paragraph.  But, Yes, I put a multi-meter from ground to various ADC pins while the Teensy had no power (not even VBat for the RTC) and my multi-meter indicated about 1500 ohms from the ADC pin to ground.  Maybe it is not linear (not a pure resistor) and could change with different voltages but I think that does clearly show the input would sink power from the battery.

--- End quote ---

But if VBat wasn't powered, that's still not quite the situation you would be in.  The test I'd like to see is with the coin cell powering the RTC, but the main power off, so that this mysterious electrical isolation is in effect, how much current flows from the coin B+ to the ADC pin, with a 10K resistor in series just to be safe.  My thought is that the coin battery may somehow be powering the isolation, such as by disconnecting its own ground from the rest of the chip.  If it does that, you won't see any current flow through ADC.  It's actually what "electrically isolated" should mean.

bitogre:

--- Quote from: Peabody on July 19, 2020, 06:21:25 pm ---But if VBat wasn't powered, that's still not quite the situation you would be in.  The test I'd like to see is with the coin cell powering the RTC, but the main power off, so that this mysterious electrical isolation is in effect, how much current flows from the coin B+ to the ADC pin, with a 10K resistor in series just to be safe.  My thought is that the coin battery may somehow be powering the isolation, such as by disconnecting its own ground from the rest of the chip.  If it does that, you won't see any current flow through ADC.  It's actually what "electrically isolated" should mean.

--- End quote ---

I suspect what is going on and causing the load is that there is probably over-voltage protection diodes to clamp in the input voltage to Vdd + the diode voltage drop.  So, with Vdd free floating, the moment you put a voltage source on the input, it tries to power the UMC with that input voltage.  Dave did a video on this:

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