Periodically reset microcontroller
The output of a 555 is normally low.  Since the minimum duty cycle for an astable 555 is 50%, you want to use a very high duty cycle (close to 100%) such that the low period is very short which will reset the microcontroller.

So you don't interfere with the existing circuit, you want to make use of an open-drain buffer, like the 74LS07.  The 100K resistor R1 will act as a pullup.  The circuit will not interfere with the power-up reset since the output of the buffer will be high-impedance when it is a logic 1.