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Old 11-05-2007, 01:34 AM
Bryan
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Default Audio Output from Spartan 3 Starter Kit

Hello,

I would like to try to get some audio output from my Spartan 3 starter
kit board, but as a hardware novice I worry about damaging my board.

Is there any risk of damage if I connect a piezoelectric speaker
directly to the pins of my FPGA? I have read that because of the high
impedance of this device it should be OK, but I would like to have a
better understanding.

Thanks,
Bryan

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Old 11-05-2007, 09:44 AM
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Default Re: Audio Output from Spartan 3 Starter Kit

>I would like to try to get some audio output from my Spartan 3 starter
>kit board, but as a hardware novice I worry about damaging my board.
>
>Is there any risk of damage if I connect a piezoelectric speaker
>directly to the pins of my FPGA? I have read that because of the high
>impedance of this device it should be OK, but I would like to have a
>better understanding.


The main danger is that mechanical shock to the speaker can produce a
high voltage at its terminals (even where your equipment is turned
off). This can damage a semiconductor device.

Does anyone here know the simplest way to avoid this? I'd guess that
a 5V zener diode across the speaker should work, but maybe there's a
better way.

(Some semiconductor devices have internal "clamp" diodes to their
power rails and this provides some protection, but an FPGA intended
for use in a "hot insertion" product probably doesn't have these
diodes).

The (electrical) impedance of the speaker depends on frequency: there
will be (mechanical) resonances at certain frequencies. Generally,
you can expect the impedance to be less when the sound is louder.
However, for a small speaker such as you might use for experiments,
even the minimum impedance is unlikely to overload an FPGA output.

Mike
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Old 11-05-2007, 11:22 PM
Ray Andraka
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Default Re: Audio Output from Spartan 3 Starter Kit

FPGAs do indeed have the protection diodes. You can drive a piezo
directly from the FPGA if you like, however you'll probably find the
audio volume less than satisfactory with the typical miniature
transducers, especially with the low voltage I/O standards found on most
of the modern FPGAs. It helps if you drive it in a differential mode.
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Old 11-06-2007, 01:12 AM
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Default Re: Audio Output from Spartan 3 Starter Kit

>FPGAs do indeed have the protection diodes...

I am guided by "Cyclone II Device Handbook, Volume 1" (Altera
Corporation, July 2005), chapter 4 ("Hot Socketing & Power-On Reset"),
page 4-1:

"There are no internal current paths from I/O pins
to VCCIO or VCCINT power supplies. Signals
driven in on I/O pins do not power the VCCIO or
VCCINT power buses".

I understood this to indicate that there are no protection diodes in
this case (except perhaps to 0V), but you indicate that there are
always such diodes. Have I misunderstood this?

Mike
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  #5 (permalink)  
Old 11-06-2007, 01:25 AM
Peter Alfke
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Default Re: Audio Output from Spartan 3 Starter Kit

On Nov 5, 5:12 pm, [email protected] wrote:
> >FPGAs do indeed have the protection diodes...

>
> I am guided by "Cyclone II Device Handbook, Volume 1" (Altera

It's the old conflict between ESD (electro-static discharge)
protection (which wants a strong diode against a well-defined voltage,
like VCC) and "Hot Socketing" which cannot tolerate a diode connected
to Vcc (since that would be against ground while the card is being
pushed in).
I cannot speak for Altera, but a traditional compromisen is a diode
against a positive Zener diode level.
If I were you, I would get a positive supply, a 10 kilohm series
resistor, and probe the pin to see how high the voltage will go. You
cannot do any harm with less than a mA, but it will clearly show you
the clamp voltage.
A good old multimeter is a wonderful thing, be it analog or
digital :-)
A scope will do also, especially when it sits already on your bench.
Peter Alfke

> Corporation, July 2005), chapter 4 ("Hot Socketing & Power-On Reset"),
> page 4-1:
>
> "There are no internal current paths from I/O pins
> to VCCIO or VCCINT power supplies. Signals
> driven in on I/O pins do not power the VCCIO or
> VCCINT power buses".
>
> I understood this to indicate that there are no protection diodes in
> this case (except perhaps to 0V), but you indicate that there are
> always such diodes. Have I misunderstood this?
>
> Mike



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