Charlieplexing 7 segment displays using Atmel Tiny26 microcontroller



Charlieplexing of discrete leds has been the topic of a few other instructables. The Charlieplexing LEDs- The theory and the How to drive a lot of LEDs from a few microcontroller pins comes to mind. They are both excellent and should be read by anyone that wants to gain a deeper knowledge of how charlieplexing really works.
Charlieplexing 7-segment displays is more or less the same as doing it with discrete leds, but with some changes to handle the fact that all the led segments have a common pin instead of being separate, and the need for buffering of the common output so the poor microcontroller can cope with the load.

Step 1 Why

While building a quick n’ dirty pulse generator that I needed to test coils for a HV power supply I decided that it’d look more funky if I used a 6-digit seven segment display instead of the ubiquitous and boring LCD display.Due to a shortage of available i/o-pins on the Atmel Tiny26 that I’ve used for the pulse generator project I couldn’t use the standard multiplexed way of doing this. The standard multiplex would require 14 i/o-pins – 8 for the segments (don’t forget the dot) plus 6 for the common anode/cathode of each display.
By Charlieplexing the displays I only need 9 i/o-pins and the displays are still muxed in a 1:6 way acheving the same brightness as standard muxing. Charlieplexing usually only light up one led at a time thus giving a reduced brightness if you want several leds to be (visibly) lit at the same time.
Of course I could have used a BCD-to-7segment decoder chip (74LS48) plus a 1-to-8 decoder (74LS138) but that would have been cheating, and i didn’t have any ’48ths as hand and I really wanted to be able to fit the pulse generator in an Altoids-like box.

Step 2 Disclaimer for bad design practices

Normally you must connect leds via a current limiting resistor in order to keep them alive for more than a few milliseconds.
Here I’m not using any resistors since the display is connected to the output pins of a microcontroller that normally can’t sink or source more than a few times the current the leds/displays can handle continuously. Since we’re multiplexing the displays they will only be on for 1/6’th of the time and can handle much more current than if they were on all the time.
In the following schematics and in my construction I’ve left out the current limiting resistors, both for the leds themselves and also the base resistor for the transistors.
Please note the following
It is a really bad construction practice to skip the current limiting and rely on luck and that duty cycle combined with the relatively low sink capability of the processor and the probable underrating of the leds maximum specs.
In anything that you do professionally, or just want to keep running for an extended period of time (longer than your first test run) one should adhere to good construction practices, read the datasheets and to the maths.
And read rgbphils comment about running leds over the rated current in the comments section of this instrucatble.

Step 3 Normal multiplexed displays

The displays I’m using here is of the Common Anode -type. That means that all the segments have a common positive pin. In order to light one segment you connect the common pin to plus and one or more of the segment pin(s) to minus.
A normal multiplexed display have all A-segments connected together in one line, and then all B-segments and so on. The common pins are then connected individually to the microprocessor. This makes eight lines for the segments and one line for the anode on each individual display.

If we instead use the Charlieplexing technique we can reduce the connection count to just 9, this regardless if we have 2, 3, 4, 5, 6 ,7 or even 8 displays.

The drawback of using Charlieplexing is that the connections to the displays is a bit more complex and the software that will scan each display one by one will also become somewhat more complex. But hey! If you can save 7 output pins on the microcontroller I think a couple of more lines of code is a cheap price. As you can see in the software step in this instructable the scanning software isn’t really that complex and can easily be implemented in your language of choice.

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