HW-179 8-Digit 7-Segment LED Display (MAX7219 Driver)

[Demon]

Hi Chuck,

Awesome work! Have you figured out how many MAX7219 we can daisy-chain?

I saw posts elsewhere mentioning a "capacitance limitation" with SPI, and that bit-banging permitted more drivers in a chain (or vice-versa).

I'm chaining 10 drivers with total wire length under 7-8 inches.




EDIT: These guys got me thinking.

https://forum.arduino.cc/t/max7219-s...ngth/250346/14


I can just as easily use 2 chains of 5 drivers. I've got the pins.

[Ioannis]

The data out of each MAX is buffered internally. I see no problem cascading here.

But LOAD (/CS) and CLOCK are connected in parallel for all chips and this may be a problem. In this case you can use a buffer like this SN74LVC1G126-Q1.

Also take a note of Cascading Drivers page 11 of the datasheet.

Ioannis

[richard]

LOAD (/CS) and CLOCK are connected in parallel for all chips and this may be a problem

after the first 1000 or so chips, the max load of a max7219 logic i/p pin is 1uA. cable capacitance is the limit [meters of it at least ]

[Ioannis]

Well, not exactly. Each input is 1 micro Amp, so 1000 is 1mA.

But still within driving capabilities of a PIC.

Cable capacitance is different matter and may lead to problems. So a buffer is not big thing to include.

Ioannis

[Demon]

How to use MAX7219 with common anode 7-segment displays:

https://forum.arduino.cc/t/max7219-a...control/118609


I'm writing lyrics for a song called 7-Segment Roller-Coaster Ride. So far my song outline goes like this:

- 2 days ago in the morning, disappointed it was so complicated to drive dozens of displays.
- 2 days ago later that day, discovers MAX7219 can drive up to 8 digits and can be chained.
- yesterday morning, finds out Digikey sells them for $20.
- yesterday afternoon, finds out JLCPCB has them in stock for $1.30.
- this morning, realizes my displays are common anode and Maxim specifically says "for common cathode".
- this afternoon, finds thread on arduino forums how to use MAX7219 "backwards" with common anode.
- tomorrow, everything crashes down cause of some other reason.

I'm thinking of having a husky singing back-up vocals.




Edit: ....AAAAAND..... POOF!

Reversing the matrix only supports numbers 0 to 7. I need to display 8 and 9 as well.


... AAAAAND ... reversal!

So these guys apparently figured out how by turning off the decode mode on the MAX7219, but someone in the comments raised a concern over current limitations (a cathode is capable of withstanding the current of 7 segments + decimal point, but reversing the polarity of the circuitry means a segment will support the current from all 8 digits):

https://www.plingboot.com/max7219-an...node-displays/