Reducing port count needed to drive 7-segment display

[ajith]

I was looking for options to reduce uC port usage to drive 7-segment displays. While the typical standard for reducing port count needed to drive LEDs is charlieplexing, it requires being able to drive both ends of each LED to one of 3 states. I had difficulty in trying to reconcile that with the common-anode or common-cathode design of available 7-segment displays.

Found this nice instructable which made me realize that i was perhaps overthinking the problem. While it is not strictly charlieplexing, it allows one to work with the common anode or cathode configuration. Ref: https://www.instructables.com/id/Charlieplexing-7-segment-displays/

The code is attached in the instructable but i expect it would be trivial for most people on here once the schematic makes sense.

In the event that instructable gets take down due to lack of traffic, here is the web archive link : https://web.archive.org/web/20190417170756/https://www.instructables.com/id/Charlieplexing-7-segment-displays/





Needless to say, I claim no credit to any of this. Just sharing this in the hope this is useful to others trying to solve a similar problem.

P.S. : This is my first post. Any errors are unintentional. Feedback appreciated.

[madhan7710]

 You can use a simple ultra low cost display driver ic to drive 7segment displays. Just 3 or 2 wires enough. Check for TM1616 or TM1650 to drive up to 4 digits. It takes care of all the scanning and you no need to refresh the display untill you update its value. No current limiting resistors, no transistors, 8 level brigtness control.. excellent chip. I have personally used it and very much satisfied.

[ajith]

Agreed. The price one pays for the lower part count is design and code time and reduced brightness. Sometimes this approach is more compelling, at other times, using a driver makes more sense.

[kripton2035]

+1 for cheap led driver. TM1638 if for driving up to 8 digits and a keyboard simultaneously.
I would use direct transistor drive only for less than 4 digits.

[jzaghal]

Why not use the 74HC595 serial to parallel shift register??

[sadman]

Why not use the 74HC595 serial to parallel shift register??

charliplexing when you want to keep your design tidy or you never want to add any chip

[ratovarius]

Another good display driver is HT16K33 from Holtek - https://www.holtek.com/productdetail/-/vg/16K33

[pushycat]

Why not use the 74HC595 serial to parallel shift register??

Yes i would suggest the same. Just using shift register behind every display and any number of displays can be driven with just 5 pins including power. For reference, have a look at this shared project

https://www.pcbway.com/project/shareproject/W14332ASZ1_Project_Outputs_for_7segment_Shift.html

[comlekciler]

I think I understand, the key point is to make "high impedance" leds that will not give light.
Can we formulate this method as follows? N*(N-1) leds can be controlled with N number of pins.

[odessa]

Doesn't get much cheaper or easier than the 74HC595, I've just made a 6 x 7 segment display using them. Dead easy to drive too.

[madhan7710]

In my opinion using a dedicated TM1616 like chip is much better than using shift register because the load on the microcontroller is zero. There is no need for scanning every n mS which you can use for other major tasks. A single chip can drive 4 (even 8 based on the chip ) displays whereelse you need 4 shift register ic's to drive 4 displays.

[Ahmad_k]

For me the MAX7219 is the best solution. With one chip you can drive 8 7segments display through SPI connection. very good brightness and very cheap , arround 0.15$ in bulk

[GeneralMadDog]

I also use MAX7219 / 7221. Good availability and cheap, ready to use displays available. But where can you get the parts so cheap? 

[Wizpic]

I've also used the Max7219 and the TM1367 to build a new volt and amp meter for my power supply, The only trouble with the MAX7219 I could only get the .31" displays modules off E-bay which is cheaper than buying the chip it's self. I could get the TM1367 modules with .56" display. I used the MAX7219 to build my DRO for my digital calipers as  it was a new design. I did try the MAX7219 modules in my PSU but it did not look right.

[Ahmad_k]

I also use MAX7219 / 7221. Good availability and cheap, ready to use displays available. But where can you get the parts so cheap? 

We have multiple dealers in my country that import electronics from china.

[bobcat1]

The question that should be asked is : "what is more expensive to use 9 uc pins or 5(or less) uc(micro controller) pin's"
The answer is depend on what is the requested final result " whether you plan to manufacture 10000 pcs of a new device or system - cost is very important or you just going to produce few hobby PCB?

All the best

Bobi

[pickit2]

at my place of work we moved away from 7 seg displays that used 7 pins plus 1 pin for each digit displayed.
we started using LCD displays 7 pins then we started using LCD and PCF8574 2pins.
the way the product developed at each stage there was better information displayed and the cost went down.
All of the above was in black and the option of a back light, and tried a few colour back lights.
Today they use TFT displays with so much for information to the user.
I think that's one of the reasons there is so many cheap 7 seg displays & driver chips on the Chinese market.
I even think in my old workplace stores there are still lots in there, mostly kept for repairing units out on sites.

so even a a hobby project I would use a TFT display, using 4 or 5 data & control pins.
and if pins are that tight use a Nextion display borrowed from the works storeroom.
 

[jzaghal]

Hi,
You mean TM1637 ??

[Wizpic]

Hi,
You mean TM1637 ??

Yeah sorry, fingers to fast for my brain

[anotherandrew]

Your schematic will not work well with NPN transistors for common anode displays. base must be able to 0.7V above emitter and with led on emitter you add 1-3V to amount of voltage base must be to turn on transistor. Just use PNP and select with LOW not high and it will work better, but you may need second (NPN) transistor to turn on PNP if display voltage is above MCU pin voltage.

[max]

Hi,

See the attached files for single n dual pin interface of hc595.

Regards

[PeteBee]

I'm using the MAX6951 for charliplexing in a design that can drive either 8x7 segment displays or up to 64 discrete LEDs.  The MAX6951 is driven from the microcontroller using SPI, so only 3 I/O pins used, plus chip select if you have more than one device on SPI (One version of my design has 2xMAX6951 for up to 128 LEDs or 16x7 seg displays). Another advantage of using Charliplexing with discrete LEDs is that you can use 2 pin bi-colour LEDs, as the drive in each direction produces one of the two colours.

[sadman]

Complete guidance how use and configure max72xx 

https://playground.arduino.cc/Main/MAX72XXHardware/