Quote Originally Posted by mackrackit View Post
Read this, it gets interesting...

http://www.picbasic.co.uk/forum/show...t=11575&page=1
Hmmmm, there was a heated debate in there . However, like DT mentioned in that thread, you can drive a regular LED (using a 3V supply) from a PIC without any problems, but if you connect that same LED directly to the power rail it will burn and it can burn your fingers (I say this from experience ).

The reason it works on the PIC is the internal resistance of the pin in the PIC. For example, the following graph is taken from the 16F84A datasheet from the AC/DC Characteristic Graphs section.

Name: VOH-Graph.JPG Views: 1844 Size: 32.2 KB

By calculating the slope in the dashed line it can be determined that the internal resistance for VOH = 3.0V is equal to ROH = 130 Ohms. Now, in the case the output is low, VOL = 0 then the VOL-IOL graph is

Name: VOL-Graph.JPG Views: 2118 Size: 38.8 KB

Again, by calculating the slope of the curve it can be determined that the internal resistance ROL = 36 Ohms.

So, in the case that you are using a regulator of 3.0V you will have an equivalent PIC resistance of 130 Ohms; therefore, there is no need for an external resistor.

More detailed information about port characteristics can be found in the next book in chapter 3.

http://www.amazon.com/Designing-Embe...0073607&sr=1-1