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I know this has been covered before on an unrelated subject, but I am
trying to understand it better now.

Let's say I have one output on a micro (AVR) that is going to connect
to the inputs of 5 other micros. I know I should put a series
resistor between them - so one 1K resistor in series at the output pin.

Now at the inputs, I would pull the signals high with a 10K resistor.

The output signal is then connected to the Micro side of the pull up
resistor at the input pin (micros).

This works when going from one micro to another. However, when going
from one micro to more than one micro (inputs), I am puzzeled as to
what to do...

I could use one pull-up connected to all the input pins. This seems
to make the most sense.

But, for some reason I keep thinking I need a pullup on each of the
input pins.

I believe both would work fine, but one way has to be "Right" on way
has to be "Wrong".

Anyone care to clarify this situation for me?

Chris

Chris,

One thing to keep in mind, if you have 1 micro connected to 5 and they all have a 10K pullup, the effective pullup is a 2K resistor.

My initial thought is that you could use a single resistor unless the distance is excessive. If it were me I'd try a single resistor, no greater than 4K7.

Cheers,

Peter.

lcdpublishing wrote:

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--- In Electronics_101@..., "lcdpublishing"
<lcdpublishing@...> wrote:

I know this has been covered before on an unrelated subject, but I am
trying to understand it better now.

Let's say I have one output on a micro (AVR) that is going to connect
to the inputs of 5 other micros. I know I should put a series
resistor between them - so one 1K resistor in series at the output pin.

Now at the inputs, I would pull the signals high with a 10K resistor.

The output signal is then connected to the Micro side of the pull up
resistor at the input pin (micros).

This works when going from one micro to another. However, when going
from one micro to more than one micro (inputs), I am puzzeled as to
what to do...

I could use one pull-up connected to all the input pins. This seems
to make the most sense.

But, for some reason I keep thinking I need a pullup on each of the
input pins.

I believe both would work fine, but one way has to be "Right" on way
has to be "Wrong".

Anyone care to clarify this situation for me?

Chris

I wouldn't think you would need a pull-up if you are driving the input
with a logic output. You would need it if you were using an open
collector circuit or a regular switch.

Thinking about this series resistor, it is really used to protect pins
from overcurrent. Whether the pin should be an input or output isn't
really the issue.

I would normally suggest a 330 ohm resistor to protect an output pin.
However, as you are driving 5 inputs and would like to protect them
from each other, the resistor value should probably be 1.5k at each
input pin. At this point I get dubious. I don't like high series
resistance but with CMOS there really won't be much voltage drop
across the resistor. I suppose even 10k would actually work.

If you find a circuit where you do need pull-up, you have to consider
where it is connected. If you did use 10k series resistance and 10k
pull-up at the pin, the lowest you could pull the pin would be 2.5V
(voltage divider) and that probably wouldn't work. If you installed
the pull-up at the outer end of the series resistor then you could
pull all the way to ground (assuming no current significant current
flow through the series resistor).

Richard


So, I would put a 330 ohm resistor in series with each of the 5 inputs
and connect them to the one output. This way you can't damage the
device with the output unless all