{"id":4321,"date":"2019-05-14T20:00:43","date_gmt":"2019-05-14T18:00:43","guid":{"rendered":"https:\/\/msalamon.pl\/?p=4321"},"modified":"2025-12-27T19:53:10","modified_gmt":"2025-12-27T18:53:10","slug":"no-more-multiplexing-on-gpio-max7219-in-action-part-2","status":"publish","type":"post","link":"https:\/\/msalamon.pl\/en\/no-more-multiplexing-on-gpio-max7219-in-action-part-2\/","title":{"rendered":"No More Multiplexing on GPIO! MAX7219 in Action, Part 2"},"content":{"rendered":"\n

Recently<\/a> we made a 7-segment display multiplex itself thanks to the magical MAX7219<\/a> chip. Today I\u2019ll show you how these chips let you handle a theoretically infinite number of displays. We\u2019re talking about the daisy-chain connection these wonderful chips offer. In a moment you\u2019ll see how, without using more pins, you can connect and control as many displays as you want. Let\u2019s get to it!<\/p>\n\n\n\n\n\n\n

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Complete series of posts:<\/p>\n\n\n\n

No more multiplexing on GPIO! MAX7219 in action part 1<\/a><\/p>\n\n\n\n

No more multiplexing on GPIO! MAX7219 in action part 2<\/a><\/p>\n\n\n\n

No more multiplexing on GPIO! MAX7219 in action part 3<\/a><\/p>\n\n\n\n

MAX7219 \u2013 daisy-chain connection<\/h1>\n\n\n\n

To increase the number of LEDs you can control, you would connect another MAX7219<\/a> chip. In the \u201cclassic\u201d setup you would allocate a separate CS pin to select the active chip, while data and clock would be connected in parallel to the second chip. That would even make sense, but what if I told you I want to connect 500 chips to drive thousands of LEDs? Find me an STM that has over 500 pins. Tough one\u2026<\/p>\n\n\n

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This is where daisy-chaining comes to the rescue. What is it, and why does it let you handle 500 chips without trouble? I\u2019m glad you asked. A cascade occurs in nature or gardening and is a type of waterfall or fountain. In general, it\u2019s about the flow of water. As a reminder, take a look at the first image from Google of such a formation.<\/p>\n\n\n\n

How does this relate to electronic circuits? Notice that a cascade is made of stages. Each such stage is one of our MAX7219<\/a> chips. The water represents the data we send into the chain. The top of the waterfall is the first chip. The data (water) flows through it and reaches the next ones, flowing through successive chips in the chain. In the electronic waterfall there\u2019s also a moment of accepting, or more formally, latching the data. After all, what we send to the chips should reach them, not just pass through. Also note that when you introduce a new item of data, all older ones shift forward by one, so when you write something for element no. 0, in a moment it will reach element no. 1. We don\u2019t want that one to perform the same task as well\u2026<\/p>\n\n\n\n

To control what the chips latch, we use the regular CS. The MAX7219<\/a> will accept the data it currently holds only if a rising edge appears on the CS pin. Returning to the waterfall metaphor, the CS could be a frog that eats the bug (the data) floating by :). What does the connection of several MAX7219<\/a> chips look like? The documentation will help.<\/p>\n\n\n

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As you can see, on the MCU side there are only three signals (on the left):<\/p>\n\n\n\n