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Content preview: On Friday 09 February 2007 04:09, Jonas Meyer wrote: >
In the thread <<Tmote Sky interface to PIC18F2431>> Rob states >
<<"unused" signals are preferably configured as outputs, outputting a >
low>>. > > Is there any reason for this behavior? According to my
understanding > "unused" pins should be configured as inputs by default.
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--- Begin Message ---
On Friday 09 February 2007 04:09, Jonas Meyer wrote:
> In the thread <<Tmote Sky interface to PIC18F2431>> Rob states
> <<"unused" signals are preferably configured as outputs, outputting a
> low>>.
>
> Is there any reason for this behavior? According to my understanding
> "unused" pins should be configured as inputs by default.
The msp430 is a CMOS device. When a pin is configured as an input, it can
draw unreasonably large currents if the voltage at the input pin is not very
close to GND or Vcc. A floating pin can have a potential that floats into
the "middle" between GND and Vcc, causing relatively large current draws.
This is why unused pins should be set to outputs.
> "Unused" pins
> could actually be "used" by an external device outputting high. This
> would force the output drivers to quit their service or at least stress
> them unnecessarily.
If the external device is always driving the net to the io pin when the msp430
is powered, then it's safe for the io pin to be treated as an input, since it
is not "unused". If the external device can come and go, for example it may
not always be present, or it may set its output to a high impedance state,
then the net will float and you run into current problems. The easiest
solution is to add an appropriate pull-up or pull-down resistor to the net
connecting the msp430's io pin and the external device.
Note that at least some of the new msp430 2xx parts have programmable pull-ups
and pull-downs. This creates more flexibility when dealing with io pins that
connect to devices that are only periodically present or driving the
respective nets.
Hope this helps,
Steve
--- End Message ---
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