Light Emitting Diode Displays

Driving 12 leds with 4 uController pins

From Mark Willis, Jeff King, Don McKenzie, Craig Lee

(A)--\/\/--+-----+-----+
           #     |     |
           #     |     |
(B)--\/\/--+--+--(--+  |
              #  #  #  #
              #  #  #  #
(C)--\/\/--+--+--+  |  |
           #        |  |
           #        |  |
(D)--\/\/--+--------+--+

Where each ## in the diagram is actually two LED hooked "anti-parallel" like this:

 |----|<-----|
-|           |-
 |---->|-----|


ABCD  LED ON
----  ------
0000  none
1111  none
zzzz  none
01zz   1
10zz   2
0z1z   3
1z0z   4
z01z   5
z10z   6
0zz1   7
1zz0   8
zz01   9
zz10  10
z0z1  11
z1z0  12

Questions:

• equant@yahoo.com asks: " In the table, I'm not sure what a 'z' represents. Is that an input pin?" James Newton replies: High impedance. A state where the pin is pulled neither high nor low but instead allowed to float.

  James Newton replies: Now that I think about it: Yes, in a microcontroller z would represent a pin that had been set as an input.