At 08:33 PM 3/1/2005, Jesse Lackey wrote:

>Some ICs I've used have configuration pins that can be tied high, low, or 
>floating, to select three different modes.
>It would be handy to be able to do this with a microcontroller.
>
>Now, if 'tied' means thru a resistor, the one way I came up with is to:
>1. read value high/low
>2. switch pin to output
>3. set it to opposite of what was read on #1
>4. switch pin to input
>5. read value high/low
>
>if done quickly, if the value at step #5 is different from the value at 
>step #1, then it is floating.  (You read back the tiny parasitic 
>capacitance the pin has that was charged in step #3).
>
>I haven't tried this, tho it would obviously be an easy test.
>
>But.  This does not strike me as reliable.  And it really has to be 
>reliable, as reliable as using normal set the pin high or low configuration.

I normally do this with a resistor tied to the pin; the resistor is driven 
from some other known signal that the micro either generates or can 
sample.  Standard setup for me is where I use a bussed SIP network feeding 
all the input pins on a shift register - this gives me 8 trinary inputs.

If you want to sample a single pin, I might try something like a 10n 
capacitor in series with a 470R resistor tied to the PIC pin; other side of 
cap is grounded.  Your jumper pins connect the junction of the capacitor & 
resistor to Vdd, Vss or nothing.  This should be reliable.  Choose your 
sample period to be at least 4 or 5 time-constants.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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