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   REPLYADDR cl@isbd.net   
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   MSGID:  4893166c   
   REPLY: <5e8745255b.DaveMeUK@BeagleBoard-xM> 8a5535c1   
   PID: SoupGate-Win32 v1.05   
   David Higton  wrote:   
   > In message <9qjq7k-sk32.ln1@esprimo.zbmc.eu>   
   >           Chris Green  wrote:   
   >   
   > > When you power down a Pi is there any simple way to protect inputs which   
   > > are being driven by externally powered equipment?   
   > >   
   > > I have A2D devices on the I2C bus which are powered from the Pi, if the Pi   
   > > has power removed the A2D's will be unhappy I think as their inputs will   
   > > still have the voltages they are measuring applied to them.   
   > >   
   > > Similarly I may have one or two logic inputs which are driven by external   
   > > signals (to see if things are turned on for example) and these inputs will   
   > > still be there (at 3.3v logic levels) when the Pi is powered down.   
   > >   
   > > Would diode clamps to the Pi's 3.3v supply be OK or are there better ways?   
   >   
   > If the external signals are strong enough to feed a pull-up that's low   
   > enough in value to be fast enough, one way is to feed the external signal   
   > into the emitter of an NPN bipolar transistor, with a pull-up from the   
   > collector to the Pi's 3.3V supply, and a pull-up from the base of   
   > something like 20 times the resistance, also to the Pi's 3.3V supply.   
   >   
   > It's unusual, but it's just a common-base saturated level shifter.   
   >   
   That's an idea, thank you, for logic inputs it'll be fine.   
      
   --   
   Chris Green   
   ·   
      
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