Raspberry Pi GPIO

1. Raspberry Pi GPIO Pin naming conventions Using sysfs Using the Wiring library Git and Github

2. Pi Overview • So far we have tried to setup a “headless” connection to your Pi in this classroom. • Serial with the FTDI cable • Ethernet on the CS LAN • WiFiin Rhodes-Robinson Hall • a hidden network managed by ITS • You must demonstrate your ability to connect to your Pi using at least two of these three methods in order to receive a passing grade in this class. • This can be done at any point through out the remainder of the semester---I will keep a list on Moodle • There are now over ten workstations configured in the CS lab (RRO 223) to support a monitor, keyboard, and mouse connection. • This is an easier to get things working properly • try it out, but put all the cables back!

3. Pi Overview • We’ve learned a little about the Pi • BCM 2835 processor, 3.3V (3V3) power on pins, SD card is like the hard drive,… • We’ve learned a little about Linux • The root directory: / , the super user designation: sudo, change permissions: chmodugo+x filename, … • We’ve learned a little about networking • ssh pi@yourLastName-pi.cs.unca.edu • ifconfig • The contents of the file: /etc/network/interfaces • Today, our focus is on using the Pi as an embedded system controller

4. Pi Setup for Today Will also need a WiFi or an Ethernet connection Connect to power adapter not the USB port of your computer • Because the cobbler connector has a notch, you can only put the cable in the right way • But, it is possible to put the cable in upside down on the Raspberry Pi Pin 1 is the colored wire-must connect to pin 1 on the Pi

5. RPi General Purpose IO (GPIO) Pins • 17 GPIO pins brought out onto the P1 header • most have alternated functions • two pins for UART; two for I2C; six for SPI • All 17 pins can be GPIO (i.e., INPUT or OUTPUT) • all support interrupts • internal pull-ups & pull-downs for each pin • I2C pins have onboard pull-ups • using them for GPIO may not work • Pins are 3.3V not 5V like on the Arduino • They are connected directly to the Broadcom chip • Sending 5V to a pin may kill the Pi • Maximum permitted current draw from a 3.3V pin is 50mA Image credit: http://elinux.org/RPi_Low-level_peripherals

6. The Bigger Picture (image credit: http://pihw.wordpress.com/2013/01/30/sometimes-it-can-be-simple) Diagram includes BCM GPIO references (GPIO.BCM), common functions, WiringPi pin references, and Pin numbers (GPIO.BOARD). A cheat nice sheet

7. Using the GPIO Pins • There are two different methods to read or write these pins using Linux • Creating a file-type access in the file system • Write/read memory addresses allocated to the GPIO peripheral of the SoC using pointers • Memory locations can be found in the datasheet for the BCM2835 • We can use the Wiring library to help with both

8. Connect an LED using a resistor between GPIO 17 (P1-11) and GND The LED will initially be off because the GPIO pins are initialized as inputs at power-on (except for TXD).

9. Using the File System • Create and run the following shell script (blink.sh) using sudo: sudo ./blink.sh #!/bin/shecho 17 > /sys/class/gpio/exportecho out > /sys/class/gpio/gpio17/directionwhile truedo        echo 1 > /sys/class/gpio/gpio17/value sleep 1        echo 0 > /sys/class/gpio/gpio17/value sleep 1done Make the pin available for other applications using with the command: echo 17 > /sys/class/gpio/unexport

10. More Detail • Create a shell script using nano: • nano blink.sh • Cut and paste the previous slide to nano window • Ctrl-w to save then Ctrl-x to exit nano • Change the permissions on blink.sh: chmod 755 blink.sh • Run blink.sh: sudo ./blink.sh (in directory where blink.sh is stored) • After running the script your LED should be blinking endlessly. Give the command: Ctrl-c Ctrl-c to abort the script • All of the commands in the script can be issued one at a time on the command line; beginning by giving the commands: sudo -i to run a root shell---notice the change in the prompt • Look at the files and their contents in directory /sys/class/gpio/ and its subdirectories --- see next slide

11. Understanding /sys/class/gpio/ • In Linux everything is a file: /dev/ttyUSB0, /sys/class/net/eth0/address, /dev/mmcblk0p2,… • sysfsin a kernel module providing a virtual file system for device access at /sys/class • provides a way for users (or code in the user-space) to interact with devices at the system (kernel) level • A demo • Advantages / Disadvantage • Allows conventional access to pins from userspace • Always involves mode switch to kernel, action in kernel, mode switch to use, and could have a context switch • Much slower the digitalWrite()/digitalRead() of Arduino

12. A C program to do the same thing • GPIO with sysfs on Raspberry Pi (Part 2) • Code on Github • Beware: the code assumes a Rev1 pinout

13. Github • The heart of GitHub is Git, an open source project started by Linux creator Linus Torvalds • Git manages and stores revisions of projects • Think of it as a filing system for every draft of a document • Git is a command line tool • GitHub provides a Web-based graphical interface • Basic functionality

14. Introducing the WiringPi library • A GPIO access library written in C for the BCM2835 • Writes/reads the base address of the memory allocated to the GPIO • Similar to the Wiring library in the Arduino used to make common IO operations easier • Features: • command-line utility gpio • supports analog reading and writing • More • Install the Wiring Pi library following these instructions

15. Wiring Pin Numbers Image credit: https://projects.drogon.net/raspberry-pi/wiringpi/pins/

16. Blinking lights with Wiring #include <stdio.h> #include <wiringPi.h> // LED Pin - wiringPi pin 0 is BCM_GPIO 17. #define LED 0 int main (void) { printf ("Raspberry Pi blink\n") ; wiringPiSetup () ; //  note the setup method chosen pinMode (LED, OUTPUT) ; for (;;) { digitalWrite (LED, HIGH) ; // On delay (500) ; // mS digitalWrite (LED, LOW) ; // Off delay (500) ; } return 0 ; }

17. Running blink • Compile and run the blink program gcc -Wall -o blink blink.c -lwiringPi  compile sudo ./blink  run • Runs forever---kill with the command ctrl-c ctrl-c • Note: One of the four wiring setup functionsmust be called at the start of your program or your program will not work correctly

18. Accessing memory allocated to the GPIO • /dev/memprovides user-level access to SoCmemory • Offset 0x20000000 is a address of BCM peripherals • wiringPi.c writes to that area of memory to control the pins

19. Controlling a Servo with the Pi • Controlling the servos requires PWM, aka Pulse Width Modulation • The Arduino does this very well, the Raspberry Pi does it less well • The Arduino program has complete control of the microcontroller • when it is running loop() nothing else can use the CPU • Except for interrupt handlers written as part of the Arduino program • On the Raspberry Pi, your program runs within a Linux OS • The Linux OS may switch to running another program! • But you can change your program’s scheduling priority • Some ways of getting the Pi to give the impression that it is a real time system and to do PWM ‘properly’: • Gordon Henderson has written about an improvement to the WiringPi library to allow threaded PWM on every GPIO pin taking up 0.1% of the CPU each • Rahul Kar has blogged about using the WiringPi library and PWM • WiringPi recommends ServoBlaster

20. Connect a Parallax Servo Servo Connector: Black – Pi’s ground Red – Pi’s 5V White – signal on GPIO 17 Image credit: http://www.parallax.com/ NOTE: For a single small servo you can take the 5 volts for it from the Pi header, but doing anything non-trivial with four servos connected pulls the 5 volts down far enough to crash the Pi

21. Using WiringPi’s servo example #include <stdio.h> #include <errno.h> #include <string.h> #include <wiringPi.h> #include <softServo.h> int main () { if (wiringPiSetup () == -1) { // setup to use Wiring pin numbers fprintf (stdout, "oops: %s\n", strerror (errno)) ; return 1 ; } softServoSetup (0, 1, 2, 3, 4, 5, 6, 7) ; // wiringPi pin numbers for (;;) { softServoWrite(0, 0) ; // wiringPipin 0 is BCM_GPIO 17 delay (1000) ; softServoWrite (0, 500) ; delay (1000); softServoWrite(0, 1000) ; delay (1000); } }

22. Running servo.c • To compile: gcc-Wall -o servo servo.c wiringPi/wiringPi/softServo.c compile softServo.c -IwiringPi/wiringPi path to softServo.c -lwiringPi include wiring library • To run: sudo ./servo • Calling softServoWrite() ; • The 1st input is the pin number • The 2nd input should be from 0 (hard left) to 1000 (hard right). • The 2nd input refers to the number of microseconds of the pulse. • An input of 0 produces a 1000uSec (1mSec) pulse (hard left) • An input of 1000 produces a2000uSec (2mSec) pulse (hard right) • An input of 500 produces a 1500uSec (1.5 mSec) pulse (stop)

23. Using the gpio utility • The programgpiocan be used in scripts to manipulate the GPIO pins • Thegpiocommand is designed to be called by a normal user without using the sudo command or logging in as root • Try at the command line: gpio mode 0 out gpiowrite 0 1 • Sets pin 0 as output and then sets the pin to high • More info on the gpio utility

24. And There’s more • WiringPi provides support for C programming • There’s a lot of support for programming in Python: • http://openmicros.org/index.php/articles/94-ciseco-product-documentation/raspberry-pi/217-getting-started-with-raspberry-pi-gpio-and-python • http://learn.adafruit.com/playing-sounds-and-using-buttons-with-raspberry-pi/install-python-module-rpi-dot-gpio