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Being fairly new to MC development, someone on this forum made the
following helpful comment regarding switches:

"On reflection, it is always a good idea to put a 330 ohm resistor in
series with switches during development. So, connect one end of the
330 ohm resistor to the input pin and the other to the switch output
(T,1,2). Connect the pull-up resistor between the switch output and
Vcc, not between the input pin and Vcc."

A few weeks into this new hobby, I've indeed managed to damage a
switch, an lpc2148 development board, and an LCD. A bit discouraging,
but ultimately no big deal as long as the lesson is learned. I was
wondering if someone would be able to give me some tips on using
resistors to avoid these kind of situations in the future, though.

For example, in which cases should I be placing a 10K resister between
a component and the pin on an MC, or are there other cases I should
use a 330 ohm resistor aside from the switch outputs mentionned above?

I suspect there are a few simple rules that can go a long way to
preventing certain problems, and if anyone can point any of them out,
I'm sure they will be welcome to interested beginner's like me.

Kevin
------------------------------------



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--- In l...@yahoogroups.com, "Kevin Townsend" wrote:
>
> Being fairly new to MC development, someone on this forum made the
> following helpful comment regarding switches:
>
> "On reflection, it is always a good idea to put a 330 ohm resistor in
> series with switches during development. So, connect one end of the
> 330 ohm resistor to the input pin and the other to the switch output
> (T,1,2). Connect the pull-up resistor between the switch output and
> Vcc, not between the input pin and Vcc."
>
> A few weeks into this new hobby, I've indeed managed to damage a
> switch, an lpc2148 development board, and an LCD. A bit discouraging,
> but ultimately no big deal as long as the lesson is learned. I was
> wondering if someone would be able to give me some tips on using
> resistors to avoid these kind of situations in the future, though.
>
> For example, in which cases should I be placing a 10K resister between
> a component and the pin on an MC, or are there other cases I should
> use a 330 ohm resistor aside from the switch outputs mentionned above?
I doubt you would ever place a 10k resistor in series with a uC input.
The 10k value would more likely be used to pull a pin high with
working with a normally open pushbutton or switch. It is best placed
at the switch end of the resistor, not the uC end.

The 330 ohm resistor limits the amount of current that can flow into
or out of a pin. Suppose a pin is define as an output and set high.
Accidentally, this pin is connected to a switch that shorts it to
ground, Maximum current will flow and this may be more than the pin
can handle. The 330 ohm resistor will limit this current to a safe value.

In summary, during development, almost all pins can be protected by
330 ohm resistors. They won't usually upset A/D signals and most
things driven by a uC don't draw much current. Thus there is very
little voltage drop across the resistor.
Richard

------------------------------------



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>> For example, in which cases should I be placing a 10K resister between
>> a component and the pin on an MC, or are there other cases I should
>> use a 330 ohm resistor aside from the switch outputs mentioned above?

You size resistors on pins depending on the pin leakage currents.

'Digital design; principles and practices;, 2nd Ed, J. F. Wakerly
has a nice discussion in Ch 3.

CMOS has low input leakage currents, so you'll find pull-up and
pull-down values around the 1k to 10k mark. Current limiting
resistors are in the 100 to 300 ohm range.

If you have to work with TTL, you'll find that they have lower
leakage currents in the high-state relative to low. That's
why you'll see higher valued pull-ups relative to pull-downs.
Its also why you see chip-selects as active low logic;
it consumes less power to hold the signals in a deasserted
high-state rather than a low state.

Cheers,
Dave

------------------------------------



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