The mraa C library is the most direct way to get in touch with the Intel Edison's GPIO lines. In this chapter we take a look at the basic operations of input and output, examine GPIO read/write timing, and learn how to use interrupt handling to improve performance when detecting changes in input readings.

This is a chapter from our ebook on the Intel Edison. The full contents can be seen below. Notice this is a first draft and a work in progress. 

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<ASIN:1871962447>

The mraa library is designed to smooth out the differences between hardware. In this case hardware includes the breakout board type as well as the CPU.

The Edison with the Arduino breakout board is complicated because some of the raw pins are used for more than one thing - they are multiplexed - and there is a lot of extra hardware connected to the raw pins for analog I/O for example. Mraa provides a simple interface to this complicated mess and make it look as if the Arduino "shield" I/O lines are simple GPIO lines. 

The mini-breakout board simply delivers the raw I/O lines to the connectors with no processing of any kind. This makes the task of the mraa library much simpler. The mraa pin numbers map directly to physical pins on the connector and correspond to simple GPIO lines. 

You can argue that the mraa library really only comes into its own when the breakout board needs to be configured as well as the raw GPIO lines but as it already exists it is an easy and standard way to work with the native Edison hardware.

In this chapter we take a first look at using mraa to control the GPIO pins as basic input output lines.  

In the next chapter we tackle the problem of making GPIO fast enough to do some "bit banging" that is low level protocols implemented in C. To do this we need to look at how to make Linux a "near" real time operating system. 

If you know all about the basics of GPIO use including the subtleties of interrupt handling then jump to the next chapter. 

Pin Numbering

The one thing that drives the Edison programmer mad is the different pin numberings in use. Basically there are three different pin numberings in use - Arduino, mraa and SYSFS. Of these the SYSFS number can be considered the "native" Edison GPIO numbering.  

The Arduino pin numbering corresponds to standard Arduino shield pins. This is not in any way directly connected to the Edison GPIO numbering because multiple physical pins are used for different purposes on the Arduino breakout board. If you are using the min-breakout or anything that is closer to the real Edison hardware you can mostly ignore Ardunio pin numbering. The only time that it might be of concern is if you are trying to convert an Arduino sketch into C when you can look up which SYSFS number the pin corresponds to and then use the table given later to look up the corresponding mraa number.

The SYSFS pin numbers are the raw hardware defined GPIO lines that the Edison provides. In many ways you can consider these numbers to be reality. Linux exposes almost all external hardware as if it was a file system - character oriented like a terminal or block oriented like a disk. The GPIO lines are also provided to the user and programmer alike as a file system with each pin corresponding to an I/O stream. You can work directly with the GPIO by using Linux file commands and the pin numbering used is the standard hardware derived numbering. 

Finally we have the mraa numbering which has nothing to do with anything as it is intended to provide a numbering that is independent of device and breakout board. Why this is an advantage is no entirely clear as most embedded programs target a specific platform and usually define things like pin numbers as constants at the start of the program. However if you are going to use mraa it is mraa's numbering you need to also use.  

You can see the pin numberings in the table below. Don't worry about Pinmode 1 for the moment. By default we are working with the I/O pins setup as Pinmode 0 - more of this later. 

In Pinmode 0 all of the lines are configured as digital I/O lines i.e. pure GPIO lines. In Pinmode 1 some of the pins have a special functions like PWM or I2C.  

You can see from the table that in the previous chapter we toggled mraa line 13 which is physically J17-14 and in Pinmode 0 Edison GPIO-128 as a SYSFS pin.

In most cases you can simply use the mraa pin number but if you want to use the Edison Pin number which corresponds to the Linux SYSFS pin number you can via the mraa_gpio_init_raw function which accepts SYSFS pin numbers. So you can change the init function to:

mraa_gpio_context pin = mraa_gpio_init_raw(128); 

and still toggle the pin connected to  J17-14.

The only reason for using SYSFS pin numbers is if you are converting a program written as a shell script.

Drive Characteristics

Each GPIO line can be configured as a general purpose I/O line.

For input the shortest pulse times that are recognized are given as:

• 100 ns for a 50 MHz clock when SoC is in S0 state.
• 260 ns for 19.2 MHz clock when SoC is in S0i1 or S0i2 State.
• 155.5 µs for 32 kHz clock (RTC) when SoC is in S0i3 State.

In most cases these smallest pulse widths are well below what can be measured using mraa. 

In output mode each GPIO line can supply or sink 3mA.