Many Thanks Gerhard, 

I will pop something on the breadboard tonight and see how it works :)

Jim



On Thu, 17 May 2007 08:58:45 -0300, Gerhard Fiedler wrote
> Vasile Surducan wrote:
> 
> > On 5/16/07, Jim <jimf@webstudios.co.uk> wrote:
> >> I am using RB4-RB6 inputs to detect changes to a couple of Normally 
closed
> >> switches and raise an interrupt. This technique I have used 
successfully in
> >> the past, But I also have a number of Normally Open switches on RC0 - 
RC5
> >> that I want to trigger a RB interrupt too. What I am proposing is to 
add a
> >> 1N4148 diode pointing to the RB input from each of the RC switches in 
DDL
> >> style - Is this likely to work? What should I watch for?
> >>
> >>
> >>
> >>
> >>             +5v
> >>            --+--
> >>              |
> >>              |
> >>              \
> >>              / Pullup R
> >>              \
> >>              /
> >>              |
> >>              |            NO Switch
> >>  To Pic  ----+-----------O  O--------+
> >>  RC Input    |                       |
> >>              |                       |
> >>            ----                      |
> >>             \/                       |
> >>            ----                     ---
> >>              |                      GND
> >>              |
> >>   To Pic   --+
> >>   RB Input
> 
> > Maybe in the configuration below, a pull down resistor on RB
> > (10K-100K) will help having a controlled current flow through diode.
> > When NO will be closed, the falling edge may be sensed on RB.
> 
> A pull-down resistor would create a voltage divider with the pull-up,
> wouldn't it? 
> 
> I'd probably use pull-ups all around and reverse the diode:
> 
>               +5V
>                |
>             pull-up1
>                |
> Normal input1 -+------switch---Gnd
>                |
>                --
>                /\ diode
>                |
> Interr input --+
>                |
>             pull-up Int
>                |
>               +5V
> 
> You can also use all pull-downs, connect the switch to positive and reverse
> the diode. Depending on the type of inputs used (ST or TTL), one or the
> other gives you a better safety margin. Using Schottky diodes also helps
> with this. But with the RB ext. interrupt input being normally ST 
> IRRC, having a Vil of 1V (at 5V Vdd), the ~0.7V it gets is fine. (Using
> pull-downs wouldn't help with ST inputs, because their margins are
> symmetrical.)
> 
> Additional inputs get their own pull-up and add another diode to the
> interrupt input. This is then what's called a "wired or" or 
> something the like.
> 
> Gerhard
> 
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