Lpc2000

Thanks Bill, I'm earwigging here for a while trying to pick up enough on 
the LPC2139 to seem as if I know what I'm doing. ;?}

Al

Bill Knight wrote:
> Al
> Yes you can, if you want to implement two interrupts per cycle.
> The project this was for had a number of interrupts firing off
> much of the time.  Because we had a conviemt capture pin, the
> decision was made to use it instead.
> 
> -Bill
> 
> On Sun, 23 Jan 2005 23:23:10 +1030, onestone wrote:
> 
> 
> Can't you just use a single timer and toggle the bits in the control
> register that set the trigger edge? I haven't studied LPC2000 in any
> great detail yet, but this is so basic every 4,8and 16 bitter I've
> worked with that had a capture channel could do this. Then just check
> the edge control bits to determine if you need a result.
> 
> Al
> 
> Bill Knight wrote:
> 
>  > We am doing something similar with two capture inputs to the
>  > same timer.
>  > - setup 1st capture input to capture the time of the rising edge
>  > - setup 2nd capture input to capture the time of the falling edge
>  >    and to cause an interrupt.
>  > - the interrupt routine saves the difference between the two as
>  >    the pulse duration.
>  > - the interrupt routine could also save the raw values for period
>  >    and duty cycle calculations (we don't need that so don't do it).
>  >
>  > Regards
>  > -Bill Knight
>  > http://www.theARMPatch.com
>  >
>  >
>  > On Fri, 21 Jan 2005 23:05:45 -0000, chazeltopman wrote:
>  >
>  > I'm trying to capture the width of a pulse using the capture
>  > timers on the LPC2106 and getting an accurate reading every
>  > time has proven difficult.  The pulses are 200 to 700
>  > microseconds long.  I thought that I could capture them in
>  > the following way:
>  >
>  > Algorithm 1:
>  >    o set up capture timer channel 0 to interrupt on each edge
>  >    o on interrupt check level to see if high to indicate a
>  >      rising edge
>  >    o if high, remember captured time and wait for falling edge
>  >    o when falling edge occurs subtract rising edge time and
>  >      queue up the length of the pulse
>  >
>  > I implemented this algorithm and my numbers vary quite a bit.
>  > If I scope the signals, they are clean so the variance is not
>  > due to noise and is due to interrupt collision or a misunderstanding
>  > of the timer capture.  The above algorithm will fail if another
>  > interrupt occurs preventing the captured count from being read
>  > before another edge transition occurs.  This is probably what is
>  > happening in my case.  So I've been thinking about how else to
>  > capture the two edges without software getting in the way.  The
>  > only way I can think of is to use two capture inputs and a match
>  > register.
>  >
>  > Algorthm 2:
>  >    o the first capture input captures the time on the rising edge
>  >    o the second capture input captures the time on the falling edge
>  >    o the match register provides a timed interrupt in which at
>  >       least two edges have been captured
>  >
>  > In my case I'd set it to be a millisecond since the pulse is
>  > periodic with about a millisecond period (I'm really measuring the
>  > duty cycle).  Then comparing the rising and falling edge capture
>  > times, I can deduce whether I've captured the low pulse (falling
>  > edge before rising edge) or the high pulse (rising edge before
>  > falling edge) and calculate the high pulse from the captured times.
>  > Also by taking a group of measurements, I should get some of
>  > the high pulse and some of the low pulse.  Using this info, I
>  > should be able to deduce the period as well instead of measuring
>  > that independantly.
>  >
>  > While this algorithm seems like it should work, it uses two inputs
>  > per pulse input.  If someone else has a better idea please toss
>  > it into the arena.
>  >
>  > Rob
>  >
>  >
>  >
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