[OpenTRV-dev] Schematic is good

[Damon Hart-Davis]

Hi,

(CCed to the list for general geeky interest, as we are starting to firm up the schematic and think about the PCB for V0.2-Arduino see https://sourceforge.net/p/opentrv/code-0/HEAD/tree/trunk/Arduino/ , Bo is asking why I have not put a supercap on the ATmega board.  I maintain the RTC in software on the ATmega328P using about 1.3uA, and there is no trivial way to keep that running while repowering the rest of the circuit, eg when replacing batteries.)

I've taken a slightly different approach.  The time is saved every 15 minutes.  (Using 1 byte of EEPROM for intraday time in a way that should be OK for 12 years continuous activity.)

Thus, on swapping batteries you lose on average 7.5 minutes, 15 at most, if you are quick putting new ones in.

Considering the potential need to make two adjustments per year of 1h for DST, a probable timing error of 1s/day (ie 10mins/year), and a battery life of a year or two, that doesn't seem too bad to me.

We *could* mess around with supercaps and diodes and so on, but this seems good enough for government work...

Rgds

Damon


On 13 May 2013, at 14:52, Bo Herrmannsen wrote:

> at this point i got a thought... how do we keep time in case power fails?