I’ve been doing run tests on the units over the last week or so taking samples every minute (simulating about 30 days in the field). Data logged well, but band-gap monitoring reported Vcc wobbling by more than half a volt once the supply fell below 3.8 volts. Not sure what to make the cutoff voltage though and I wonder if that monitoring code is a bit flaky? Even so, the accelerometer ran smoothly with very little noise on the axis readings during the wobbling voltages. But I did see a bit of downward drift in the internal temperatures being reported by the RTC, and the accelerometer. Not sure what caused that.
The whole unit is showing about 60mv drop on the 3 AA no-name battery pack per 100 readings. That’s about a days worth of operation at 15 minutes sample intervals, which projects out to about 45 days of operation on a 6 pack before I hit 2.7 v (my current best guess at a ‘safe’ minimum voltage for SD card writes). Not great performance I admit, but good enough for some field testing anyway.
And a last minute batch of new boards arrived today from TinyCircuits, so I will have at least one full set of spare parts in the box if I need it…Sweet!
Q: I wonder what would happen if I let the little loggers run right down to the point where the internal brown out detector kicks in and shuts down the whole system? Would this hurt the SD card? No time for destructive tests now though, as I am on the plane to Mexico tomorrow morning.
Q: I wonder what other data I could be reading out of that bma250 accelerometer? If it’s got an internal bump detector, does that mean that it’s internally calculating theta on the fly? In which case I could read it out directly over I2C as well?
Q: I still have that old mma7361 lying around that I’d like to put in service, but I’m not sure how stable an analog device would be with an unregulated power supply?
A: I found a discussion of the issue over at avrfreaks. It looks like they the relative measurements stay the same, so if you are recording vcc, you can do the correction.