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REPLYADDR dave@davehigton.me.uk
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MSGID: <5e8745255b.DaveMeUK@BeagleBoard-xM> 8a5535c1
REPLY: <9qjq7k-sk32.ln1@esprimo.zbmc.eu> 29fa7b6c
PID: SoupGate-Win32 v1.05
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.
David
--- SoupGate-Win32 v1.05
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