Need 2 ms hardware timer

I am looking to implement a 2 ms hardware timer function. Condition is the
Input goes from High to Low and stays Low indefinitely. Output should be a
2m positive pulse.
I looked at the 555 timer, but the trigger input must be a negative pulse
for the timer to work properly. This is a hardware implemented fail-safe
function to monitor the output of the MPU just in case the firmware get
lost. The internal MPU watchdog will not do the job that I need.

TIA,
David