Controlling Lots of Outputs from a Microcontroller

Making LED displays is fun. There are a a few tools that get used all the time, from row-column scanning to LED driver chips to multplexers and shift registers. This tutorial discusses some of the more popular methods for controlling large amounts of LEDs from a microcontroller, including their various strengths and weaknesses, and how they work. For more on this subject see chapter 14 of “Physical Computing“, where Dan O’Sullivan and I discussed it in more depth.  I’ll also include some notes on how to apply these ideas to controlling multiple motors or other high-current loads.

Most microcontroller modules have a limited number of outputs. Even if you use the analog inputs as digital I/O, there are only 19 pins on an Arduino, for example. That’s a fairly typical number for an 8-bit controller, and it seems not nearly enough if you want to control, say, 100 LEDs or more.  There are a couple ways around this problem.  Without adding any additional hardware, you can make a matrix of your LEDs and control them using row-column scanning.  If you want discrete analog control over one output at a time, you can use a multiplexer. For digital control over multiple pins, you could use an addressable latch or a shift register. If you need pseudo-analog control over multiple pins, you could use a PWM driver.  There are also several LED driver chips that are designed specifically to control groups of LEDS.

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STP16C596 Shift Register

This tutorial will show how to control multiple LED outputs from a microcontroller using an STP16C596 shift register. The STP16C596 is similar to the popular 74HC595 shift register, but it’s nicer because it can sink a constant current to the LEDs it’s driving. It works slightly differently, however, so this code won’t work exactly for the ‘595.

This is a stub. More explanation will follow, but for now, here are schematics and code for Arduino.

Parts you’ll need:

  • STP16C596 shift register
  • Arduino microcontroller
  • LEDs
  • 1-kilohm resistor

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