Exploring Edison - Fast Memory Mapped I/O
Written by Harry Fairhead   
Wednesday, 23 September 2015
Article Index
Exploring Edison - Fast Memory Mapped I/O
Fast GIPO access
Fast Input
Complete program; Summary

 

The complete program with the necessary includes and initialization is:

#include <stdio.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/stat.h>


uint8_t* load_memmap();

int main()
{

 mraa_init();
 mraa_gpio_context pin31 = mraa_gpio_init(31);
 mraa_gpio_dir(pin31, MRAA_GPIO_OUT);
 
 load_memmap();

 int pin= 44;

 uint32_t mask=(uint32_t)(1 << (pin % 32));
 uint8_t valoffsetOn = 0x34;
 uint8_t valoffsetOff = 0x4c;
 
 uint8_t* loc=(pin / 32) * sizeof(uint32_t)+ mmap_reg;


 for (;;) {
  *(volatile uint32_t*) (loc + valoffsetOn) = mask;
  *(volatile uint32_t*) (loc + valoffsetOff) = mask;
 }
 return MRAA_SUCCESS;
}

uint8_t* load_memmap(){
 int mmap_fd;
 struct stat fd_stat;
 uint8_t* mmap_reg;
 mmap_fd = open(
  "/sys/devices/pci0000:00/0000:00:0c.0/resource0",
  O_RDWR);
 fstat(mmap_fd, &fd_stat);
 mmap_reg =(uint8_t*) mmap(NULL, fd_stat.st_size,
   PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED,
   mmap_fd, 0);
 return mmap_reg;
}

 

Notice that pin mraa pin 31 is SYSFS pin 44.

All of this isn't very difficult but it is fairly pointless. Most of the pulses produced are about 0.2 microseconds with the occasional pulse around 0.25 microseconds. However at this speed the pulse shape is poor and capacitive effects start to make a difference to the high and low times. If you are using a logic analyzer then the pulse width you measure depends on the  thresholds it uses.

There seems to be only a small overhead in using the mraa read/write functions. 

Notice also that as the memory mapping isn't organized into registers you can't make use of it to set or unset multiple pins at a time. 

Summary

  1. Using standard SYSFS I/O the Edison works with signals in the 20 to 50 microsecond region.

  2. Using fast memory mapped I/O you can work with signals in from 0.25 microsecond output with around 5 microsecond inputs.
     
  3. For fast I/O busy wait is the only way to achieve these higher speeds.
     
  4. Fast I/O not only allows you to work with faster pulses it also permits better synchronization between switching multiple I/O lines. 

  5. You can make use of direct read/writes to the memory mapped drivers and by pass mraa but the gains are small.  

Where Next

Now we can work with much faster signals it is time to tackle those troublesome glitches. We need to find out how to use Linux in a  near real time mode - see the next chapter.

More Information

The starting point for finding out about all Intel's Internet of Things resources, including Edison, is the Intel IoT Developer Zone.

 

Exploring Edison

ExploringEdison

 

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Chapter List

  1. Meet Edison
    In this chapter we consider the Edison's pros and cons and get an overview of its structure and the ways in which you can make use of it. If you have ever wondered if you need an Edison or an Arduino or even a Raspberry Pi then this is the place to start. 

  2. First Contact
    When you are prototyping with the Edison you are going to need to use one of the two main breakout boards - the Arduino or the mini. This chapter explains how to set up the Edison for both configurations. 

  3. In C
    You can program the Edison in Python, JavaScript or C/C+ but there are big advantages in choosing C. It is fast, almost as easy as the other languages and gives you direct access to everything. It is worth the effort and in this chapter we show you how to set up the IDE and get coding. 

  4. Mraa GPIO
    Using the mraa library is the direct way to work with the GPIO lines and you have to master it. Output is easy but you do need to be aware of how long everything takes. Input is also easy but using it can be more difficult. You can use polling or the Edison interrupt system which might not work exactly as you would expect.

  5. Fast Memory Mapped I/O
    There is a faster way to work with GPIO lines - memory mapped I/O. Using this it is possible to generate pulses as short at 0.25 microsecond and read pulse widths of 5 microseconds. However getting things right can be tricky. We look at how to generate fast accurate pulses of a given width and how to measure pulse widths.

  6. Near Realtime Linux 
    You need to be aware how running your programs under a non-realtime operating system like Yocto Linux effects timings and how accurately you can create pulse trains and react to the outside world. In this chapter we look the realtime facilities in every version of Linux. 

  7. Sophisticated GPIO - Pulse Width Modulation 
    Using the PWM mode of the GPIO lines is often the best way of solving control problems. PWM means you can dim an LED or position a servo and all using mraa. 

  8. Sophisticated GPIO -  I2C 
    I2C is a simple communications bus that allows you to connect any of a very large range of sensors. 

  9. I2C - Measuring Temperature  
    After looking at the theory of using I2C here is a complete case study using the SparkFun HTU21D hardware and software. 
     
  10. Life At 1.8V
    How to convert a 1.8V input or output to work with 5V or 3.3V including how to deal with bidirectional pull-up buses.

  11. Using the DHT11/22 Temperature Humidity Sensor at 1.8V 
    In this chapter we make use of all of the ideas introduced in earlier chapters to create a raw interface with the low cost DHT11/22 temperature and humidity sensor. It is an exercise in interfacing two logic families and implementing a protocol directly in C. 

  12. The DS18B20 1-Wire Temperature 
    The Edison doesn't have built in support for the Maxim 1-Wire bus and this means you can't use the very popular DS18B20 temperature sensor. However with a little careful planning you can and you can do it from user rather than kernel space. 

  13. Using the SPI Bus 
    The SPI bus can be something of a problem because it doesn't have a well defined standard that every device conforms to. Even so, if you only want to work with one specific device it is usually easy to find a configuration that works - as long as you understand what the possibilities are. 

  14. SPI in Practice The MCP3008 AtoD 
    The SPI bus can be difficult to make work at first, but once you know what to look for about how the slave claims to work it gets easier. To demonstrate how its done let's add eight channels of 12-bit AtoD using the MCP3008.

  15. Beyond mraa - Controlling the features mraa doesn't. 
    There is a Linux-based approach to working with GPIO lines and serial buses that is worth knowing about because it provides an alternative to using the mraa library. Sometimes you need this because you are working in a language for which mraa isn't available. It also lets you access features that mraa doesn't make available. 
 

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More Information

Intel IoT Developer Zone

Intel Edison Starter Guide

Intel Edison Software Download

XDK Intel IoT

Forum Intel Edison Get Started

Intel System Studio

Sensors for Iot Projects

mrra on GitHub 

Related Articles

Powerful New Features For Edison 

Intel Announces IoT Platform       

 

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Last Updated ( Tuesday, 10 May 2016 )