connecting several LEDS to uC output

[purplewolf]

Hello,
I am new to uC programming..I have to connect approximately 70-80 LEDs to PIC microcontroller ..but PIC has limited number of output channels..i am looking for efficient solution to do it...its a simple problem but i am not getting it what to use and how to do???
plz guide me urgently

 

[betwixt]

The usual method if the LEDs are part of a display is to use multiplexing or 'charlieplexing'. You connect the LEDs in rows and columns. One connection from each row goes to a pin in the micro and one connection from each column goes to a pin n the micro. You only light the applicable LEDs in one row or one column at a time but you move rows/columns so quickly that persistence of vision gives the impression they are all working simultaneously.

You can control 80 LEDs with only 18 pins if you use multiplexing.

If you use 'charlieplexing' you can reduce the pin count even further but the controlling gets more complicated. You connect LEDs between pins using both polarities, for example if you have pin A and pin B, you connect two LEDs head-to -tail parallel between them. If A is high and B is low, one LED light, if A is low and B is high the other LED lights and if A and B are the same the LED does not light.

Alternatively, if you want to use a hardware solution, look at devices like the MAX7219 which can drive 64 LEDs and can be chained so two devices could drive 128 and so on. they communicate serially with the microprocessor on only three wires.

Brian.