I need a circuit to act as a watchdog to a micro (which is not powered (instead of in a sleep mode)most of the time).
it needs to be low power (< ~4uA) and non-programmable. So no I2C RTCs or MCUs.
I've looked at RC timers combined with a comparator but extremely large resistors and caps become too expensive for the timeout required (25hours).
I've also looked at using shift registers to divide out an astable multivibrator to the correct time, but ICs (counter/dividers/flip-flops) i've been able to find draw too much current - ~8uA alone.
Answer
If the current draw specification can be extended to 5 μA from the specified 4, the CSS555C micropower timer, a distant cousin of the classic 555 timer, can be used for the purpose.
The CSS555C is a hybrid device, with analog timing circuitry including integrated timing capacitor, a 6-decade counter/divider, and an on-board EEPROM to store the counter settings. The decade divider allows the CSS555C to provide a single delay of the order of days.
The diagram below illustrates this extended timer operation:
Specifically, note the "C" at the end of the part number: This indicates the variant with an internal capacitor integrated on the chip, thus reducing current consumption further.
Incorporating this part into the design would meet the specified design parameters, of not modifying the existing microcontroller portion, and not using a separate microcontroller. The part can be powered from 1.2 Volts to 5.5 Volts DC.
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