Lpc2000

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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