With 5V directly applied to an input on the Propeller, the VDD substrate diode will be stressed.
The PN junction of the diode only wants to "see" a maximum of 0.4V More than this and you will
prematurely shorten the life of the Propeller.
To solve this, all you need is a current limiting resistor in series with the I/O. This in turn will form
a "voltage divider" with the substrate diode.
In most applications a 1K 3.3k resistor will work just fine limiting the current to 1.3mA 394uA from a 5V input.
Where you might want to consider another option is if the input speed is very high. The I/O pins
also exhibit a small capacitance which in conjunction with an input resistor form a low pass filter.
In other words a very high frequency on the input "might" get filtered out and not seen by the Propeller.
Comments
With 5V directly applied to an input on the Propeller, the VDD substrate diode will be stressed.
The PN junction of the diode only wants to "see" a maximum of 0.4V More than this and you will
prematurely shorten the life of the Propeller.
To solve this, all you need is a current limiting resistor in series with the I/O. This in turn will form
a "voltage divider" with the substrate diode.
In most applications a 1K 3.3k resistor will work just fine limiting the current to 1.3mA 394uA from a 5V input.
Where you might want to consider another option is if the input speed is very high. The I/O pins
also exhibit a small capacitance which in conjunction with an input resistor form a low pass filter.
In other words a very high frequency on the input "might" get filtered out and not seen by the Propeller.
Refer to this previous discussion....
http://forums.parallax.com/showthread.php?p=586894
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 5/24/2006 5:35:00 AM GMT