Re: a basics question: using input pins, pullup, short to ground vs driven signal.
- From: jleslie48 <jon@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 28 Apr 2009 17:59:19 -0700 (PDT)
On Apr 28, 8:46 pm, Muzaffer Kal <k...@xxxxxxxxx> wrote:
On Tue, 28 Apr 2009 17:30:35 -0700 (PDT), jleslie48
<j...@xxxxxxxxxxxxxxxxxx> wrote:
My question is if I want to take my driven output and run it around as
an input signal, how do I do that?
Are you saying you want to connect an output pin to an input pin and
leave the pushbutton connected so that either can drive the input?
Make the output pin so called "open collector" that only drives low and
floats high.
outputpin <= '0' when mysignal = '0' else 'Z;
-Jeff
the part that confuses me, is the 3.3 volts that I see sitting on the
input pin when its not tied to ground. how can I send "in" from the
external wire 3.3 volts when it already has 3.3 volts on it?
If you consider that there is always a connection to the input pin, I
think it would clear up your issue. If you have a pull-up with 3.3V
and the input agrees with it, you are sending in a high signal. If
your external driver disagrees with the pull-up and drives a 0, you're
sending in a low signal. If you had a 3rd state where you didn't know
if a driver is connected then you might need another signal to detect
if there is a driver or only if the pull-up is connected but I don't
think that's your configuration (actually even in that case, with a
true tri-state IO you can detect if a driver is "driving" or not).
Does that help?
--
Muzaffer Kal
DSPIA INC.
ASIC/FPGA Design Services
http://www.dspia.com
ahh! so if I connect these two wires, the input (at a steady 3.3
volts) and the other wire (that changes between 0 and 3.3 volts) that
is a normal thing to do. When both wires are at 3.3 volts I'll get one
signal on the input pin, and when one is at 3.3 (the input pin) and
what its connected to is at 0 volts (the external wire) the external
wire acts just like the closing the switch scenario aka, connecting
the pin to ground, and thus changes the state on the input pin from 0
to 1 (or 1 to 0, whatever.) It that how it works?
.
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