Arduino Uno Power Down mode with running timer

Question

for my Longtime data logger project I need to use the "power down" mode as much as possible. But I whant to save the time with every measurment data to the SD card. Besides the watchdog-timer all other timers are down at power-down mode. Is there a way to use the watchdog timer to keep track of the time or is there another solution I need to use?

Thanks

Answer 1

I think that THIS is the kind of project where RTC ICs should be used... You wake up at any time and ask him "what's the time?" and hw answers with all you need to know. And then you can forget it.

In my projects sometimes i used Maxim's RTCs. For instance the DS1339C has an embedded crystal (approx 10ppm accuracy) and a battery input (so you can also shut it down completely and he continues to track the time). From the battery input it drains less than 1uA, so the battery can last long.

I think that for a datalogger this is the best solution..

Answer 2

You could use the WDT if you are not overly concerned with accurate timing. The watchdog interrupt can be configured to wake up the device every 8 seconds or so. The SRAM will stay current, so you could keep a (volatile) counter and update it in the interrupt handler.

Even the best clock sources are subject to drift, just not as much as the WDT.

The actual frequency of the watchdog oscillator depends on the voltage of the device and the temperature. Here are the characteristics for the 2560: