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David Evennou
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 Posted: Wed Jun 20, 2007 4:02 am Post subject: Need 2 ms hardware timer |
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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
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| Posted: Thu Mar 20, 2008 9:40 am Post subject: Re: Need 2 ms hardware timer |
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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
[/quote]
Take a look at a 74121 (TI, etc). It has both L-H and H-L inputs.
....jerry |
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petrus bitbyter
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| Posted: Fri Mar 21, 2008 4:02 am Post subject: Re: Need 2 ms hardware timer |
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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
[/quote]
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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