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.
Kibalo
Posts: 9
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