FastGPIO library for Arduino

Version: 1.0.2
Release date: 2015 Mar 16
www.pololu.com

Summary

This is a C++ library for the Arduino IDE that allows you to manipulate general-purpose I/O (GPIO) pins on your Arduino or Arduino-compatible board with minimal overhead. Many function calls to this library will be inlined and compiled down to just one or two assembly instructions.

This library is intended to be a replacement for the default Arduino I/O functions (pinMode, digitalWrite, and digitalRead) and has several advantages over them:

The compiled code runs faster.
Operates only on the specific bits used by the pin instead of doing read-write-modify operations on entire registers.
Never disables interrupts.
Supports reading and restoring the state of a pin, which is useful if the pin is used for multiple purposes.
Pins can be specified either by their AVR pin name or their Arduino pin name.
Supports pins PD5 and PE2 on the ATmega32U4; these pins do not have an official Arduino pin number so they cannot be used with the default Arduino I/O functions.

To use this library, you will need a basic understanding of microcontroller I/O pins. General information on this topic can be found in the Arduino Digital Pins tutorial.

Currently, this library requires that the pin number you specify must be known at compile time. This means the pin number cannot come from a variable that might change and it cannot be the result of a complicated calculation. This allows the compiler to generate optimal code for manipulating that pin.

The Arduino GPIO functions automatically turn off a pin's PWM function when accessing it with digitalRead or digitalWrite. This library does not have that feature.

Supported platforms

This library is designed to work with the Arduino IDE versions 1.0.x, 1.5.x, and 1.6.x, and will probably not work with earlier versions.

This library currently supports any board based on the ATmega32U4 or ATmega328P.

Getting started

Download the fastgpio-arduino library from GitHub, decompress it, and drag the "FastGPIO" folder into the "libraries" subdirectory inside your Arduino sketchbook directory. You can view your sketchbook location by selecting File->Preferences in the Arduino environment. If there is not already a "libraries" folder in that location, you should create it yourself. After installing the library, restart the Arduino environment. Example code for using this library can be found in the File->Examples menu.

Specifying a pin

There are three ways to specify a pin:

By its pin number
By its AVR pin name, using the IO_* macros that this library provides. For example, IO_D3 refers to pin PD3.
By alternative names defined in the Arduino IDE's pins_arduino.h header, such as A2 and MOSI.
By the special IO_NONE macro. Commands operating on the IO_NONE pin will have no effect.

Any pin number that works with the standard Arduino I/O functions will work with the FastGPIO library.

For a full list of all the supported pins, see the "Pin number reference" section below.

Digital outputs

This example shows how to set pin 13 as an output and drive it high:

FastGPIO::Pin<13>::setOutput(HIGH)

Digital inputs

To configure a pin as a digital input with the internal pull-up resistor disabled, run:

FastGPIO::Pin<12>::setInput();

To configure a pin as a digital input with the internal pull-up resistor enabled, run:

FastGPIO::Pin<12>::setInputPulledUp();

Then, to read the value of the input, you can use:

bool inputHigh = FastGPIO::Pin<12>::isInputHigh()

Pin number reference

Pins for ATmega328P boards

Number	AVR pin macro	Alternative name
0	IO_D0
1	IO_D1
2	IO_D2
3	IO_D3
4	IO_D4
5	IO_D5
6	IO_D6
7	IO_D7
8	IO_B0
9	IO_B1
10	IO_B2	SS
11	IO_B3	MOSI
12	IO_B4	MISO
13	IO_B5	SCK
14	IO_C0	A0
15	IO_C1	A1
16	IO_C2	A2
17	IO_C3	A3
18	IO_C4	A4, SDA
19	IO_C5	A5, SCL
20	IO_C6	A6,

Pins for ATmega32U4 boards

Number	AVR pin macro	Alternative name
0	IO_D2
1	IO_D3
2	IO_D1	SDA
3	IO_D0	SCL
4, 24	IO_D4	A6
5	IO_C6
6, 25	IO_D7	A7
7	IO_E6
8, 26	IO_B4	A8
9, 27	IO_B5	A9
10, 28	IO_B6	A10
11	IO_B7
12, 29	IO_D6	A11
13	IO_C7
14	IO_B3	MISO
15	IO_B1	SCK
16	IO_B2	MOSI
17	IO_B0	SS
18	IO_F7	A0
19	IO_F6	A1
20	IO_F5	A2
21	IO_F4	A3
22	IO_F1	A4
23	IO_F0	A5
30	IO_D5
31	IO_E2

Speed test

We compiled the following code with the Arduino IDE 1.5.8 and ran it on an ATmega32U4 running at 16 MHz:

#include <FastGPIO.h>
#include <util/delay.h>

void setup() {
  FastGPIO::Pin<A1>::setOutputLow();
}

void loop() {
  FastGPIO::Pin<A1>::setOutputValueHigh();
  FastGPIO::Pin<A1>::setOutputValueLow();
  _delay_us(10);
  digitalWrite(A1, HIGH);
  digitalWrite(A1, LOW);
  _delay_us(10);
}

The pulses generated by the FastGPIO calls were 0.125 μs long because each call was compiled to just a single assembly instruction, while the pulses generated by digitalWrite were 5.1 μs long.

Error messages

If you specify an invalid pin number, GCC will produce error messages like this:

warning: array subscript is above array bounds
warning: asm operand 0 probably doesn't match constraints
error: impossible constraint in 'asm'

This is because the pin number is checked at compile time and must be valid.

Documentation

For complete documentation of this library, including many features that were not mentioned here, see the FastGPIO.h file documentation from https://pololu.github.io/fastgpio-arduino.

Version history

1.0.2 (2015 Mar 16): Improved the speed of toggling a pin's output value.
1.0.1 (2015 Jan 06): Introduced some casts to fix compiler warnings.
1.0.0 (2014 Dec 11): Original release.

microsat / fastgpio-arduino