One thing to watch out for is asymetrical speed--the
open collector drivers have different rise times
relative to the fall times. This may matter for SPI
communication at the higher speeds.
But, open collector outputs are pretty hardy, and easy
enough to replace if abused. They also afford cheap
fast voltage conversion, going higher/lower in I/O
voltage levels.
On the input side, often a simple series resistor will
suffice. Alot of IC's that I've seen state that an
input can be overdriven by up to 30mA into the pin--so
a 1k resistor will get up to 30V protection! This
works by using the ESD structures in (most) IC's to
clamp that pin voltage to +/- a diode drop of either
supply.
Now, I've done this; but others have recommended to
not depend upon the ESD structures for DC
currents--they are meant for short duration overload
conditions, not continous "abuse". They would
recommmend using external diodes and the same series
resistor. Your choice.
Me, on a board I once used series 2.2k resistors with
220pF caps to ground (high noise enviroment) to drive
a 74HCT541 buffer. The drivers were TTL, and I use
CMOS as a rule of thumb. Worked well, and ended a
long standing failure mode (damage to downstream
IC's). No diodes. The only thing I had to watch out
for was rise/fall times; HC and HCT devices will show
a spike in ICC for slow switching speeds due to both
transistors being on, unless if it is a Schotkey input
type.
Shawn
--- lcdpublishing <lcdpublishing@...> wrote:
Hi Guys,
I will be making a small I/O board for some AVRs.
As the inputs
should be pretty straight forward, I am not overly
worried about
them at the present (most of them will be switched
to ground).
However, for the outputs, I don't yet know what I
will be connecting
to them. It could be any of the following:
Mosfets, Relays, LEDs,
etc.
I have some experience with the ULN2803 and being
that I am familiar
with it, I want to use it again. But, that doesn't
expand my
knowledge much either - which is bad.
I would like both the inputs and outputs to be
fairly robust, but
certainly don't want to build them like a tank
either.
So, if you all have some suggestions, I am all ears.
But, as usual,
I am very much on my 'learners permit' :-)
Chris
Shawn Upton, KB1CKT
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