Driving 16-segment displays with MAX7219

While the code for this in theory makes sense to me, I can't find any examples online so I wanted to check to see if the wiring actually works.

I've used the MAX7219 for both 7-segment displays, and 8x8 pixel grids, so have an fair understanding of how it works.

here is the datasheet

https://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf

If my understanding is correct, the SEG go HIGH, while the DIG function as Sinks. Normally it would allow it to run 8, common cathode, 7-segment displays.

The single digit 16-segment display characters I have have 18 pins total. 16 segments, the decimal, and the common cathode.

Would it work to wire the first 8 segments to SEG-A -> SEG-DEC and then the cathode to Dig 0. Then the remaining 8 segments to SEG-A -> SEG-DEC again, and then also connect Dig 1 to the cathode.

doing this would theoretically allow 4 16-segment displays per MAX7219.

I realize I would have to write my own bytecode patterns to send to the MAX7219 for each character. I also realize I won't be able to use the decimal points.

Does this work? Or Am I missing something? The part I'm most unsure about is if it is problem to use 2 digits on the same cathode.

• "Am I missing something?" Links to the datasheets and a schematic maybe? Aug 20, 2018 at 17:20
• @Transistor added Aug 20, 2018 at 17:37
• That's a wiring diagram rather than a schematic. We might be able to make some sense of it but ... Let's see. Aug 20, 2018 at 17:50

simulate this circuit – Schematic created using CircuitLab

Figure 1. "That's not a schematic. This is a schematic."

Why it would work

If the schematic of Figure 1 is what you intend then (without reading the whole datasheet) you might be in with a chance. Page 5 says about pins DIG 0–DIG 7:

Eight-Digit Drive Lines that sink current from the display common cathode. The MAX7219 pulls the digit outputs to V+ when turned off. The MAX7221’s digit drivers are high-impedance when turned off.

So the 7219 digit pins can't be connected in parallel (unless you add a diode into each line) but the 7221 can as the pin goes high impedance when not driving a digit.

You would need to run the chip in "No Decode Mode" to allow individual segment addressing. See page 8.

Why it wouldn't work

Looking at the schematic it is now clear that when SEG A is driven high that both 'A' and 'K' will illuminate if either DIGIT 1 or DIGIT 2 is low. (Schematic beats wiring diagram.)

Since you have only one common cathode you are beaten.

• Ah great. Makes sense!! Thank you for deciphering my question. I'll look into the 7221. Looks like the other alternative is to go for a constant sink Led driver chip and use common anode digits instead Aug 20, 2018 at 18:23
• Also im aware of ICs than run common cathode 16 segment displays, they are just overly expensive for my application (lots of characters, but few refreshes) Aug 20, 2018 at 18:24
• Your main learning here should be seeing the benefit of taking the time to draw an adequate schematic. Once done the "scheme" of things becomes clear as do sneak paths, etc. Aug 20, 2018 at 18:29
• Very much agree and will take that with me. One of the problems with us computer background guys is we like to build fast and break things because the computer tells us what's wrong, which I've discovered in EE isn't the best approach. Thanks again for the help Aug 20, 2018 at 18:31
• @martinF if digit pins that are off go high, could you explain why the 7221 wouldn't work? Aug 20, 2018 at 18:59