Need 2 ms hardware timer

[David Evennou]

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

 

[neon]

just tie pin 2 hi with 10k add a series cap if any transition gors kow pin 2 will follow triggering the lm555 as oneshoot

 

[Jerry R]

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

Take a look at a 74121 (TI, etc). It has both L-H and H-L inputs.

....jerry

 

[petrus bitbyter]

Take a look at a 74121 (TI, etc). It has both L-H and H-L inputs.

...jerry

Hmm... 74121 and the like act on edges but do not wait "indefinitely" before
providing the output pulse. So you have to define how long "indefinitely"
lasts and need two timers: One to act on the edge and wait "indefinitely",
the second to produce the 2ms pulse when "indefinitely" is over and the
input is still low.

petrus bitbyter