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?

--
Chris Green
·

On 19/01/2024 16:15, 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.

??? If they are powered from the pi, how can they still have voltage?
?? if they are on the I2C bus, how are they connected to the A2D inputs
on the Pi?
>
> 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?
>

I suspect those already exist internally.
It is more relevant to prevent those getting smacked with high current.

a small resistor would limit any current

--
The higher up the mountainside
The greener grows the grass.
The higher up the monkey climbs
The more he shows his arse.

Traditional

In message <9qjq7k-sk32.ln1@esprimo.zbmc.eu>
Chris Green <cl@isbd.net> 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

The Natural Philosopher <tnp@invalid.invalid> wrote:
> On 19/01/2024 16:15, 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.
>
> ??? If they are powered from the pi, how can they still have voltage?
> ?? if they are on the I2C bus, how are they connected to the A2D inputs
> on the Pi?

They are ADs1116 I2C devices on the I2C bus. Their VDD suppy comes
from the Pi, their inputs come from whataver they are measuring which
doesn't get turned off whe n the Pi is turned off.

> >
> > 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?
> >
>
> I suspect those already exist internally.
> It is more relevant to prevent those getting smacked with high current.
>
> a small resistor would limit any current
>
Yes, the ADS1115 has internal diode clamps but it does say in the
datasheet that external Schottky diode clamps are recommended.

--
Chris Green
·

David Higton <dave@davehigton.me.uk> wrote:
> In message <9qjq7k-sk32.ln1@esprimo.zbmc.eu>
> Chris Green <cl@isbd.net> 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
·