Charlieplexing

Although it seems impossible you can drive 12 LEDs with 4 microcontroller pins and with more pins available you can drive even more LEDs! The diagram below shows the basic idea using diodes and 3 microcontroller pins to drive 6 LEDs.

The technique was invented at Maxim by a person whose name is Charlie so they chose to call the technique Charlieplexing! A play on words from the standard multiplexing method.

Some Maxim ICs that use this technique are (all have serial SPI interface as the control method):

MAX6950	Common-cathode display driver - up to 5 x 7seg
MAX6951	Common-cathode display driver - up to 8 x 7seg
MAX6954	Drives up to 16 digits 7-segment, 8 digits 14-segment, 8 digits 16-segment, or 128 discrete LEDs
MAX6955	Drives up to 16 digits 7-segment, 8 digits 14-segment, 8 digits 16-segment, or 128 discrete LEDs and kas 5 GPIO port expander.
MAX6958	4-Digit, 9-Segment LED Display Drivers with Keyscan 8 switch detects.
MAX6959	4-Digit, 9-Segment LED Display Drivers with Keyscan 8 switch detects - Debounces Up to Eight Switches with n-Key Rollover .

The fundamental concept is that a microcontroller pin can have three states:

• Output High.

• Output Low.
• High impedance or tristated i.e. set as an input.

The most important state is the high impedance state (tristated) since that isolates a node from a mesh network.

Since an LED is a diode when you reverse the voltage across a it no current flows which means no light is output from it.

Simple case 2 pins 2 LEDS

Here's a simple two LEDs example:

 

You can easily see that each LED is set on and off by setting one pin high and the other low.

D1 on P1 low, P2 high.
D2 on P1 high, P2 low.

Drive 6 LEDs using 3 pins

It gets more interesting when you add a third microcontroller pin:

If you set P3 as an input then D3 D4 D5 and D6 are disconnected (no current can flow) so it's the same circuit as before.

Truth Table operation for 3 pin charliplexing

The operation is easy to see using a truth table

Port Pin State				Diode active
P1	P2	P3		D1	D2	D3	D4	D5	D6
0	0	0		0	0	0	0	0	0
0	1	X		1	0	0	0	0	0
1	0	X		0	1	0	0	0	0
X	0	1		0	0	1	0	0	0
X	1	0		0	0	0	1	0	0
0	X	1		0	0	0	0	1	0
1	X	0		0	0	0	0	0	1

X = tristate or input.

Note: If you wire up the circuit wrongly then the current paths change so lots of LEDs will light up!

Formula for the number of LEDs

This formula tells you how many LEDs you can control with N microcontroller pins:

LEDs = N * (N-1)

Two pins gives 2 x (2-1) = 2
Three pins gives 3 x (3-1) = 6
Four pins gives 4 x (4-1) = 12
Five pins gives 5 x (5-1) =20
Six pins gives 6 x (6-1) =30
Seven pins gives 7 x (7-1) =42

Charlieplexing

Advantages

Very few pins control many LEDs.

Disadvantages

Only certain sets of LEDs can be turned on at the same time.

The way round this is to do 'normal' multiplexing but you have to ensure that you refresh the LEDs at a fast enough rate and obviously the more LEDs to be serviced means that the task is more difficult €“ requiring higher speed processing.

As well as this upping the processing speed means less current is delivered to the LED so you may need high brightness LEDs.

Ideal use

It is ideal for systems where you need to turn only one LED on at a time e.g. State indicator LEDs.

It's also not too bad for turning a few other leds on at the same time but you have to carefully select your LEDs as you are limited to which ones you can use.

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