Lpc2000

At 11:05 PM 1/21/05 +0000, you wrote:
>  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

As I remember there was a discussion of this a while ago.  The upshot was 
it was impossible to read the actual level of a pin set up for input capture.

>  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.


Actually you shouldn't need a another timer to make this work.  As long as 
both capture channels run off the same timer then
         - on rising edge
                 - time from last rising edge gives you period
                 - time from last falling edge gives you low duty time
         - on falling edge
                 - time from last falling edge gives you period
                 - time from last rising edge gives you high duty time.

As a nice benefit you period and duty cycle can be updated four times for 
every period.  Filter to suit.

Robert


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