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Science Forum Index » Electronics Forum » Need 2 ms hardware timer
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| David Evennou |
 Posted: Tue Jun 19, 2007 6:02 pm |
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Guest
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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 |
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| Jerry R |
| Posted: Wed Mar 19, 2008 11:40 pm |
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"David Evennou" <devennou@comcast.net> wrote in message
news:JuWdndV_t94W_eXbnZ2dnUVZ_r2onZ2d@comcast.com...
Quote: 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 |
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| petrus bitbyter |
| Posted: Thu Mar 20, 2008 6:02 pm |
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"Jerry R" <phx_coyotes@kawks.nyet> schreef in bericht
news:Q_uEj.23178$CD7.16810@newsfe11.phx...
Quote:
"David Evennou" <devennou@comcast.net> wrote in message
news:JuWdndV_t94W_eXbnZ2dnUVZ_r2onZ2d@comcast.com...
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
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 |
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