my near-finished is intended to be very low power and most of the time in sleep. It uses standalone ATMega328p with internal RC oscillator as clock source and 32kHz crystal connected to TC2 in asynchronous mode to keep time. This way the average power consumption is around 2uA which is fine for running from a coin cell battery for years. I want to add a Watchdog to be safe. But ATMega’s Watchdog needs 4uA, twice as much as the rest of the application! I need an external Watchdog but I cannot find a reasonable choice. Only suitable IC I know about is TPL5010 (and others in TPL5xxx family) but they are unavailable from stores with reasonable shipping costs (or eBay). So I am afraid I need to build the circuit from discrete components. But Googling for a hint did not give me much. My requirements are:
- Timeout more than 6s (wake up interval). It may be much longer and voltage, temperature dependent as long as it is more than 6s under all conditions (i.e. 6s @80°C, 5V and 1 hour @-20°C, 3V is fine).
- Power consumption < 1uA (better <0.1uA) as long as the Watchdog is being fed. The consumption may rise when the WDT triggers.
- Minimal components count (small board space) is preferred. The ATMega has about 10 unused I/O pins, they may be used to help.
Do you know a circuit that may be used for such task?