[Microbit-Python] micro bit pin drive current

[David Whale]

That is true, but unfortunately it is only part of the story.

It's not the pin drive current that could be the problem, it's the
opportunity for an inductive load (a speaker) to cause back-emf that is
above the specified pin voltage of the GPIO pins. Although I haven't done
the maths or looked at a scope trace to see how high in voltage the
back-emf peaks are at given frequencies, it might still be ok in practice
without any coupling or protection given the range of frequencies in use..

However, I spoke to Nicholas Tolervy about this resource, and he uses a
powered speaker such as this:

https://www.amazon.co.uk/gp/product/B0039YP37E/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

connected with one of these:

https://sciencescope.uk/product/bbc-microbit-sound-cable-2/


I also use one of these...

http://www.maplin.co.uk/p/kitsound-pocketboom-portable-bluetooth-speaker-green-n54dc

Which, incidently, has a nice bass response that works really well with
this fab Python resource!!

https://www.microbit.co.uk/musicfest/that-bass

It's very loud with some powered speakers, in my experience, so in some
environments you should consider a volume control of some sort!

Hope this helps.

David.




___________________________________________________________
David Whale, B.Sc (Hons), MIET
*Software Engineer and IET Schools Liaison Officer, Essex*


On 31 August 2016 at 07:23, Damien George <damien.p.george at gmail.com> wrote:

> There are no additional buffers on the pins.  The microcontroller will
> source/sink its maximum current and then the output voltage will begin
> to drop to keep the current at that maximum (and so act like a current
> regulator/limiter).
>
> On 31 August 2016 at 15:48, fresh.bread via Microbit
> <microbit at python.org> wrote:
> > I'm a bit worried by the diagram on the documentation for the micropython
> > speech module showing a loudspeaker connected directly to pins 0 and 1
> (see
> > http://microbit-micropython.readthedocs.io/en/latest/
> tutorials/speech.html)
> >
> > What is the maximum drive current for the pins. The processor chip only
> > seems to be able to sink/source 0.5ma (or 5ma in high current drive mode)
> > .Are there additional buffers connected to pins 0,1,2 that allow a higher
> > current?
> >
> > Jim
> >
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