What happens when the power goes out?

[SanG]

Thank you for the input emjay.  But my question remains: Why only R3 moteino turns back on but not other two? Also why R6 and R4 turns back on only when the supply starts at 1.8V or higher? Why R3 turns back on even when the supply starts at lower voltages such as 0.5V? is this how R3 version is designed/built and not R4 and R6?

[Felix]

I don't believe the atmega328p is supposed to work at 0.5V 
And there are only very minor differences between versions. Mind that if you're talking about the regular Moteinos (16mhz), you should not expect miracles at voltages lower than 3.3V. Even if slightly "overclocked" at 16mhz/3.3V it works great at that voltage. That is why the BOD is set to 2.7V for 16mhz operation, and 1.8V at 8mhz.

[SanG]

What I meant by "..R3 turns back on even when the supply starts at lower voltages such as 0.5V.." is that Step 1: when the R3-moteino (that I have) is ON at first, Step 2: then the Vin is slowly (or rapidly) reduced to 0.5V, Step 3: then increased it again (slowly or rapidly) to 2.7V or higher, it turns back on no matter how many times I do it. But the same thing does not happen for the other two versions (R4 and R6) I have. Please note that all the moteinos I have should have deafult 2.7V (16 MHz) fuse settings I believe because I haven't done any change to the firmware yet. My question is why is this happenning? Why R3 is turning back on while the other two are not? If it is not the version, still why one moteino is turning back on while the other two are not?

[Felix]

The R3 and R4 are virtually identical apart from the radio SMD footprints. Perhaps the passives were a different brand, maybe a different rating I'm thinking mostly the caps is where that might matter in some way.
I'd expect at least your R4 to behave the same so .. I'm not sure why.

[SanG]

Thank you for your help. I will keep you updated if I get any interesting result.

[Felix]

You did not mention if apart from the atmega and radio module, there is the 4MBIT FLASH-MEM chip on any of them?
Also, the RFM69 modules, what types are they on each of the Moteinos? That may make a difference, since over the years the HW/HCW modules have been manufactured with a different RF switch chip (the 6 pin SC70 package).
To really compare apples to apples, all these Moteinos should have identical configuration, and ideally without a radio module, or if the RFM69 is present - either from the same batch (this is impossible) or of the same type (ideally RFM69W or CW which don't have that RF switch).

[SanG]

Yes all the three moteinos mentioned do have 4MBit Flash MEM.
R3: RFM69CW - 868/915Mhz,
R4: RFM69HW - 868/915Mhz, and
R6: RFM69CW - 868/915Mhz

Note:
- R6 is the brand new one which I soldered the antenna/header pins before I used it for the first time.
- R3 is the one which always turns back on (at 2.7V or higher) no matter how low its Vin pin is at before it is increased using PSU directly or with Scaps.
- R4 and R6 turn back on (at 2.7V or higher) only when their Vin pin is at or above 1.8V before it is increased using PSU directly or with Scaps.

Thanks.