Trying to understand multiplexing

John,

The circuit you have is commonly referred as a charlie plexing.

You have the basics of pin 1 set high and pin 2 as low. However, to operate properly, P3 must be tri-stated. That is, it is neither 1 nor 0. This is accomplished different ways for different microcontrollers. I am not that big of an expert on Arduino, so I will not go into that.

For Charlie Plexing to operate correctly, the microcontroller must have truly tri-state able ports.

Steve

There's an article on Charlieplexing at Wikipedia, with a section on the circuit you've shown.

I agree with John you should read the Wki help but here is another quick quide that should help explain it.

using three pins and six LEDs.

Lets's say you have three LED control lines.  Call them A, B, and C.  Connect a pair of back-to-back LEDs between A and B, between B and C, and between A and C.

Now start by setting pins A and B as outputs, and C as an input.

Turn on LED AB1 by setting high on pin A and a low on pin B.  Turn on LED AB2 by reversing the polarities of A and B.

Set pin B as an input and leave A as an output.  Set pin C as an output.

Turn on LED AC1 by setting high on A and a low on C.  Turn on LED AC2 by reversing the polarities of pins A and C.

Set pin A as an input, and B as an output.  Leave pin C as an output.

Turn on LED BC1 by setting high on B and a low on C.  Turn on LED BC2 by reversing the polarities of pins B and C.

I agree with John you should read the Wki help but here is another quick quide that should help explain it.

using three pins and six LEDs.

Lets's say you have three LED control lines.  Call them A, B, and C.  Connect a pair of back-to-back LEDs between A and B, between B and C, and between A and C.

Now start by setting pins A and B as outputs, and C as an input.

Turn on LED AB1 by setting high on pin A and a low on pin B.  Turn on LED AB2 by reversing the polarities of A and B.

Set pin B as an input and leave A as an output.  Set pin C as an output.

Turn on LED AC1 by setting high on A and a low on C.  Turn on LED AC2 by reversing the polarities of pins A and C.

Set pin A as an input, and B as an output.  Leave pin C as an output.

Turn on LED BC1 by setting high on B and a low on C.  Turn on LED BC2 by reversing the polarities of pins B and C.

Thanks for the explanation Nate.  I'll play around with it tomorrow and see if I can do something with it.

What I am working on is a binary coded decimal clock.  I think they make a good conversation piece when guests come over but mainly I thought it would be a good learning project.  I am new to both Arduino and the 'C' language but I think I am getting a handle on the 'C'.  The clock would require 20 LEDs for seconds, minutes and hours. I was using the UNO but it does not have enough pins, hence the idea of multiplexing or Charlieplexing.  I do have a Mega 2560 as well but I am using port manipulation to control the pins and while I know that on the UNO, PORTD controls pins 1-7 and PORTB controls pins 8-13, I can't seem to locate much info for the ports on the Mega2560. I would prefer to use the UNO anyway.

Thanks again for the example.  Much appreciated.

John

Hi,

if you're looking for a pin mapping of the mega2560

About the few pins on Uno, the use of shift register could help you maybe you can give it a try

Good luck.

Thanks for the pin mapping.

I am trying to figure out which PORT controls which pins so that I can use port manipulation rather than digitalWrite().

Seems to be a lot of infor regarding this on the UNO but very little on the Mega2560. 

Thanks again.

John

I've just watched some confusion on the map i gave you or maybe an old rev of the board

if you want which Port control which pin (and that's the goal of pin mapping) take the schematic given by the arduino main site

the link to the schematic page.

http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf

I've tested it and that one worked fine.

Good luck.

Ok, I got the pin map from the link that you provided.

Thank you very much.  That should help a lot.

John