Electronics > Projects, Designs, and Technical Stuff
Clock Battery Monitor Circuit
bitogre:
I thought of another possibly simpler option: op amp as buffer. An op amp in a non-inverting unity gain should provide a high impedance (even when it does not have power) yet provide low impedance output to drive the ADC. Does this sound reasonable?
Ian.M:
An OPAMP's input impedance when unpowered depends on its exact topology and input protection network and is rarely specified.
Peabody:
I don't know about the opamp, but can I back up and ask how you plan to power up the Teensy periodically to check the battery voltage? Is the coin cell going to power that process, or are you going to temporarily manually connect the 12V supply?
In a way, using the built-in RTC seems to complicate things somewhat. I assume the RTC shares a common ground with the rest of the processor. If it didn't until it's time to measure, I don't think the ADC input would be a current sink at all.
Is there a tutorial on the Teensy RTC? I didn't find much on Youtube.
Peabody:
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?
bitogre:
--- Quote from: Peabody on July 19, 2020, 04:52:26 pm ---I don't know about the opamp, but can I back up and ask how you plan to power up the Teensy periodically to check the battery voltage? Is the coin cell going to power that process, or are you going to temporarily manually connect the 12V supply?
In a way, using the built-in RTC seems to complicate things somewhat. I assume the RTC shares a common ground with the rest of the processor. If it didn't until it's time to measure, I don't think the ADC input would be a current sink at all.
Is there a tutorial on the Teensy RTC? I didn't find much on Youtube.
--- End quote ---
I will be powering the Teensy when I am using the RV (about every other month). So, I will only be checking the RTC battery about every other month when I am using the RV (probably when I am packing it for my trip). No, the coin cell is NOT going to power that process.
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.
The info on how to use the Teensy 4.X RTC is the same as using the RTC on the Teensy 3.X (older Teensys didn't have an RTC). The documentation is minimal and the best is found in the code examples but https://www.pjrc.com/teensy/td_libs_Time.html#teensy3 has the basics (though it does not seem to have been updated for Teensy 4.x).
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