Re: [avr-gcc-list] external interrupt use

[Richard Urwin]

On Wednesday 25 Feb 2004 5:45 am, Jamie Morken wrote:
> >
> >               INT0                INT1
> >        pullup  |                   |   pulldown
> > +5 <---/\/\/\--o-|>|---o-----|>|---o---/\/\/\--------> GND
> >
> >                |      signal 1     |
> >
> >                o-|>|---o-----|>|---o
> >
> >                       signal 2
>
> I don't see the purpose of doing this as it only works with 2
> signals, and ties up two external interrupts.  If you have three
> signals with 6 diodes, and two signals low and one high, then switch
> to two high and one low, the INT0 and INT1 lines won't detect this
> change I don't think.

Theoretically what you really want to do is impossible. You cannot 
compress the information on six inputs down to two without losing 
something.

You could add a AVR1200 to pre-process the inputs, (only one edge 
triggered interrupt line, so you'd lose some timing, but only about a  
microsecond,) or add TTL logic to do it, (six XORs six RC pairs and one 
six input OR.) Which do you want?

The TTL version may still cause problems with race conditions. The 
AVR1200 could be programmed to avoid them, but would maybe need more 
I/O.

To get fast and accurate you could use a built-for-the-purpose interrupt 
controller, but that is likely to tie up many more I/O.

-- 
Richard Urwin

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