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digitalWrite(pin, HIGH); // sets 'pin' to high
The code is much more difficult for you to debug and maintain, and is a lot harder for other people to understand. It only takes a few microseconds for the processor to execute code, but it might take hours for you to figure out why it isn't working right and fix it! Your time is valuable, right? But the computer's time is very cheap, measured in the cost of the electricity you feed it. Usually it is much better to write code the most obvious way.
The code is less portable. If you use digitalRead() and digitalWrite(), it is much easier to write code that will run on all of the Atmel microcontrollers, whereas the control and port registers can be different on each kind of microcontroller.
It is a lot easier to cause unintentional malfunctions with direct port access. Notice how the line DDRD = B11111110; above mentions that it must leave pin 0 as an input pin. Pin 0 is the receive line (RX) on the serial port. It would be very easy to accidentally cause your serial port to stop working by changing pin 0 into an output pin! Now that would be very confusing when you suddenly are unable to receive serial data, wouldn't it?
So you might be saying to yourself, great, why would I ever want to use this stuff then? Here are some of the positive aspects of direct port access:
You may need to be able to turn pins on and off very quickly, meaning within fractions of a microsecond. If you look at the source code in lib/targets/arduino/wiring.c, you will see that digitalRead() and digitalWrite() are each about a dozen or so lines of code, which get compiled into quite a few machine instructions. Each machine instruction requires one clock cycle at 16MHz, which can add up in time-sensitive applications. Direct port access can do the same job in a lot fewer clock cycles.
Sometimes you might need to set multiple output pins at exactly the same time. Calling digitalWrite(10,HIGH); followed by digitalWrite(11,HIGH); will cause pin 10 to go HIGH several microseconds before pin 11, which may confuse certain time-sensitive external digital circuits you have hooked up. Alternatively, you could set both pins high at exactly the same moment in time using PORTB |= B1100;
If you are running low on program memory, you can use these tricks to make your code smaller. It requires a lot fewer bytes of compiled code to simultaneously write a bunch of hardware pins simultaneously via the port registers than it would using a for loop to set each pin separately. In some cases, this might make the difference between your program fitting in flash memory or not!
DanBackslide wrote:My steampunk ghost detector is controlled by an Arduino. Lights, a meter and a hobby servo (inside the big meter), a "power switch" to put it all in a quiet mode, and a hidden button to make it detect a ghost.
I just got a Smokenator 1000, and now I have this idea to build a temperature sensing rig using a thermocouple and some kind of display. And maybe an Xbee module to pipe the temp data inside, to another display. And maybe to a logger, so I can graph the temp...
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