Friday, October 21, 2011

Tutorial: Arduino Port Manipulation

This is chapter forty-three of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – a series of articles on the Arduino universe. The first chapter is here, the complete series is detailed here. Any files from tutorials will be found here.

Welcome back fellow arduidans!

In this article we are going to revisit the I/O pins, and use what is called “Port Manipulation” to control them in a much faster manner than using digitalWrite(). Why?

Speed! Using this method allows for much faster I/O control, and we can control or read groups of I/O pins simultaneously, not one at a time;

Memory! Using this method reduces the amount of memory your sketch will use.

Once again I will try and keep things as simple as possible. This article is written for Arduino boards that use the ATmega168 or ATmega368 microcontrollers (used in Arduino Duemilanove/Uno, Freetronics TwentyTen/Eleven/EtherTen, etc). My Arduino Mega is out at the moment, so I will update the tutorial for Mega users when it is replaced.

First, we’ll use the I/O as outputs. There are three port registers that we can alter to set the status of the digital and analogue I/O pins. A port register can be thought of  as a special byte variable that we can change which is read by the microcontroller, therefore controlling the state of various I/O ports. We have three port registers to work with:

  • D – for digital pins seven to zero (bank D)
  • B – for digital pins thirteen to eight (bank B)
  • C – for analogue pins five to zero (bank … C!)

Register C can control analogue pins seven to zero if using an Arduino with the TQFP style of ATmega328, such as the Nano or Freetronics EtherTen). For example:

It is very simple to do so. In void setup(), we use

DDRy = Bxxxxxxxx

where y is the register type (B/C/D) and xxxxxxxx are eight bits that determine if a pin is to be an input or output. Use 0 for input, and 1 for output. The LSB (least-significant bit [the one on the right!]) is the lowest pin number for that register. Next, to control a bank of pins, use

PORTy = Bxxxxxxxx

where y is the register type (B/C/D) and xxxxxxxx are eight status bits – 1 for HIGH, 0 for LOW. This is demonstrated in the following example:

// Example 43.1 // tronixstuff.wordpress.com/tutorials > chapter 43 // John Boxall - October 2011 // Digital 0~7 set to outputs, then on/off using port manipulation  void setup() { DDRD = B11111111; // set PORTD (digital 7~0) to outputs }  void loop() { PORTD = B11110000; // digital 4~7 HIGH, digital 3~0 LOW delay(1000); PORTD = B00001111; // digital 4~7 LOW, digital 3~0 HIGH delay(1000); }

It sets digital pins 7~0 to output in void setup(). Then it alternates turning on and off alternating halves of digital pins 0~7.

At the start I mentioned that using port manipulation was a lot faster than using regular Arduino I/O functions. How fast? To test the speed of port manipulation vs. using digitalWrite(), we will use the following circuit:

… and analyse the output at digital pins zero and seven using a digital storage oscilloscope. Our first test sketch turns on and off digital pins 0~7 without any delay between PORTD commands – in other words, as fast as possible. In the image below, digital zero is channel one, and digital seven is channel three:

Wow – check the frequency measurements – 1.1432 MHz! Interesting to note the longer duration of time when the pins are low vs. high. Next we do it the normal way, using this sketch:

// Example 43.2 // tronixstuff.wordpress.com/tutorials > chapter 43 // John Boxall - October 2011 // Digital 0~7 set to outputs, then on/off using digitalWrite()  void setup() { for (int a=0; a<8; a++) { pinMode(a, OUTPUT); } }  void loop() { for (int a=0; a<8; a++) { digitalWrite(a, HIGH); } for (int a=0; a<8; a++) { digitalWrite(a, LOW); } }

And the results:

That was a lot slower – we’re down to 14.085 kHz, with a much neater square-wave output. Could some CPU time be saved by not using the for loop? We tested once more with the following sketch:

// Example 43.3 // tronixstuff.wordpress.com/tutorials > chapter 43 // John Boxall - October 2011 // Digital 0~7 set to outputs, then on/off using individual digitalWrite()   void setup() { for (int a=0; a<8; a++) { pinMode(a, OUTPUT); } }  void loop() { digitalWrite(0, HIGH); digitalWrite(1, HIGH); digitalWrite(2, HIGH); digitalWrite(3, HIGH); digitalWrite(4, HIGH); digitalWrite(5, HIGH); digitalWrite(6, HIGH); digitalWrite(7, HIGH); digitalWrite(0, LOW); digitalWrite(1, LOW); digitalWrite(2, LOW); digitalWrite(3, LOW); digitalWrite(4, LOW); digitalWrite(5, LOW); digitalWrite(6, LOW); digitalWrite(7, LOW); }

and the results:

A small speed boost, the frequency has increased to 14.983 kHz. However you can now understand the benefits of using port manipulation. However there are a few things to take note of:

  • You can’t control digital pins 0 and 1 (in bank D) and use the serial monitor/port. For example if you set pin zero to output, it can’t receive data!
  • Always document your sketch – take pity on others who may need to review it later on and become puzzled about wchich bits are controlling or reading what!
Now to waste some electron flows by blinking LEDs. Using the circuit described earlier, the following sketch will create various effects for someone’s enjoyment (download).

And here it is in real life:

Now to use the I/O pins as inputs. Again, it is very simple to do so. In void setup(), we use

DDRy = Bxxxxxxxx

where y is the register type (B/C/D) and xxxxxxxx are eight bits that determine if a pin is to be an input or output. Use 0 for input. The LSB (least-significant bit [the one on the right!]) is the lowest pin number for that register. Next, to read the status of the pins we simply read the byte:

PINy

where y is the register type (B/C/D).

So if you were using port B as inputs, and digital pins 8~10 were high, and 11~13 were low, PINB would be equal to B00000111. Really, that’s it!

Now for another demonstration using both inputs and outputs. We will use a push-wheel switch from Chapter 40 on our inputs (digital pins 8~11), and a seven segment LED display for output (on digtal pins 7~0 – segments dp then a~f). The following sketch reads the input from the switch, which returns 0~9 in binary-coded decimal. This value is then used in the function void disp() to retrieve the matching byte from the array “segments”, which contains the appropriate outputs to drive the seven segment LED display unit. Here is the sketch (download).

And the ubiquitous demonstration video:


By now I hope you have an understanding of using port manipulation for your benefit. With a little effort your sketches can be more efficient in terms of speed and memory space, and also allow nifty simultaneous reading of input pins.

So have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

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