{"id":486,"date":"2009-05-28T02:38:54","date_gmt":"2009-05-28T07:38:54","guid":{"rendered":"http:\/\/www.tigoe.net\/pcomp\/code\/?p=486"},"modified":"2009-05-31T02:49:25","modified_gmt":"2009-05-31T07:49:25","slug":"controlling-lots-of-outputs-from-a-microcontroller","status":"publish","type":"post","link":"https:\/\/www.tigoe.com\/pcomp\/code\/arduinowiring\/486\/","title":{"rendered":"Controlling Lots of Outputs from a Microcontroller"},"content":{"rendered":"

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<\/a>“, where Dan O’Sullivan and I discussed it in more depth.\u00a0 I’ll also include some notes on how to apply these ideas to controlling multiple motors or other high-current loads.<\/p>\n

Most microcontroller modules have a limited number of outputs. Even if you use the analog inputs as digital I\/O<\/a>, 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.\u00a0 There are a couple ways around this problem.\u00a0 Without adding any additional hardware, you can make a matrix of your LEDs and control them using row-column scanning<\/strong>.\u00a0 If you want discrete analog control over one output at a time, you can use a multiplexer<\/strong>. For digital control over multiple pins, you could use an addressable latch<\/strong> or a shift register<\/strong>. If you need pseudo-analog control over multiple pins, you could use a PWM driver.\u00a0 There are also several LED driver chips<\/strong> that are designed specifically to control groups of LEDS.<\/p>\n

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Basic Principles<\/h2>\n

Before getting into the details of the actual methods, there are a few principles that will make your life easier.\u00a0 If you already understand these, feel free to skip to the next section.<\/p>\n

Bit Manipulation<\/h3>\n

All of the methods for output control mentioned below share one common process: you store the states of the pins in variables, as arrays of bits, and manipulate the bits to turn things on or off.\u00a0 For example, a row of eight LEDs in a shift register can be represented by an eight-bit (i.e. a byte) variable. If every other LEd were turned off, the byte would look like this:<\/p>\n

0 1 0 1 0 1 0 1<\/pre>\n
It doesn’t matter what the decimal value of this byte is, because all you care about is which bits are on and which bits are off.\u00a0 To change the states of the LEDs, you manipulate the bits.<\/p>\n
Computer variables<\/a> are stored in arrays of transistors that can be turned on or off; in other words, in binary arrays. At the lowest level of abstraction, these arrays of bits are manipulated using various logic operations, like AND, OR, NOT, and XOR.\u00a0 They’re sometimes called bitwise operators<\/strong> because they operate by manipulating the bits. Here’s how they work:<\/p>\n