Light Emitting Diodes, or LED's, are discrete compo-nents able to produce light when a current passes
through them. Most microcontroller designs use one or
more LED's. This application highlights the utility of
driving multiple LED's with a minimum number of I/O
pins. Typically, each I/O drives or sources a single LED.
To drive more than one, a high I/O count is required. In
order to reduce I/O requirements, LED's are multi-plexed in a matrix (as found on a keyboard). The com-plementary LED drive method proposes to implement
even more LEDs while using fewer I/O.
LEDs are polarized and can only operate when current
flows from anode to cathode (unlike a switch). We can
therefore take advantage of this fact. Table 1 shows the
number of possible LEDs with respect to the number of
I/O pins required. Fifty-six LEDs can be driven using
only 8 pins. The only drawback is that only one LED can
be driven at a time.M TB029
Complementary LED Drive
THEORY OF OPERATION
Author: Jean-Claude Rebic
Pioneer-Standard Some microcontrollers available today can sink high
current, while others offer a limited number of pins to
source high current. Microchip microcontrollers have a
INTRODUCTION very exible pin structure. When a pin is congured as
an input, th……
