BeagleBone Black: Analog Input Pins

Sources

http://en.wikipedia.org/wiki/Analog-to-digital_converter
http://www.linux.com/learn/tutorials/787511-how-to-get-analog-input-on-the-beaglebone-black
http://hipstercircuits.com/reading-analog-adc-values-on-beaglebone-black/

Introduction

An Analog-to-Digital Converter (ADC, A/D, or A to D) is a device or chip that converts a continuous physical quantity (analog signal) to a digital number that represents the quantity's amplitude.  Typically an ADC converts an analog voltage to a digital voltage.  This tutorial will go over the ADC pin properties and how to enable them on the BeagleBone Black (BBB).  

Analog to Digital Converter Layout

The BBB ADC pins are labelled as AIN pins and are located on the P9 header under the DC barrel jack.  

Figure 1:  BeagleBone Black GPIO pinout.

Table 1:  ADC pin names and numbers.

Name	Header	Number
AIN0	P9	39
AIN1	P9	40
AIN2	P9	37
AIN3	P9	38
AIN4	P9	35
AIN5	P9	36
AIN6	P9	33
GNDA_ADC	P9	34

Analog to Digital Properties

The BBB ADC can sink an maximum of 1.8V.  You will break the pin and likely the board if you provide an ADC more than 1.8V!  Since you will likely be measuring voltages greater than 1.8V, you can use a voltage divider to cut the voltage down.  

  

Figure 2:  Voltage divider circuit and equation.

A voltage divider is just a ratio of resistor 2 (Z2) to the total resistance (Z1 + Z2).  You can think of it in percentages.  If we want to cut down from a 5V source to 1.8V, then with a couple resistors we get:

Vin     =  5V

Z1      =  1000 Ω  =  64%

Z2      =  1777 Ω  =  36% 

Z1 + Z2 = 2777 Ω  =  100%

Using the equation in Figure 2 we find Vout =  1.8V, which is 36% of Vin!

Enabling ADC Pins

To enable the ADC driver to read the pins type the either of the following into the terminal.

 echo cape-bone-iio > /sys/devices/bone_capemgr.*/slots  

or

 echo BB-ADC > /sys/devices/bone_capemgr.*/slots  

You should be able to enable the drivers on boot by appending the uEnv.txt file with

 optargs=quiet drm.debug=7 capemgr.enable_partno=BB-ADC  

or

 optargs=quiet drm.debug=7 capemgr.enable_partno=cape-bone-iio  

However, I have had no luck with this so I just echo every time the BBB gets power-cycled.

Reading ADC Pins Via Terminal

To read the ADC pins from the terminal use the cat command.  The following command will read ADC 1 (AIN1). 

 cat /sys/devices/ocp.2/helper.11/AIN1  

You should see a value between 0-1799 representing milliVolts.  If you ground the pin, the value won't likely go all the way to 0 but that is O.K.

If you are reading the pins faster than about 250ns you could run into bogus values.  The ADC can only operate so fast because it must convert from a continuous signal to a digital one with discrete time steps.

Reading ADC Pins Via C Code

The following code enables the ADC pins inside the code (so you don't have to remember to do it!), reads the 7 ADC pins and prints the values to the terminal.  

 #include <stdlib.h>  
 #include <stdio.h>  
 #include <string.h>  
 #include <unistd.h>     //close()  
 #include <fcntl.h>     //define O_WONLY and O_RDONLY  
 #define MAX_BUF 64     //This is plenty large  
 
 //Function declarations  
 int readADC(unsigned int pin);  
 
 //main program  
 int main()  
 {  
      //Enable ADC pins within code  
      system("echo BB-ADC > /sys/devices/bone_capemgr.*/slots");  
      
      //Read ADCs  
      int adc0 = readADC(0);  
      int adc1 = readADC(1);  
      int adc2 = readADC(2);  
      int adc3 = readADC(3);  
      int adc4 = readADC(4);  
      int adc5 = readADC(5);  
      int adc6 = readADC(6);  
      
      //Print ADC readings  
      printf("ADC 0: %d\n",adc0);  
      printf("ADC 1: %d\n",adc1);  
      printf("ADC 2: %d\n",adc2);  
      printf("ADC 3: %d\n",adc3);  
      printf("ADC 4: %d\n",adc4);  
      printf("ADC 5: %d\n",adc5);  
      printf("ADC 6: %d\n",adc6);  
      
      return 0;  
 }//end main  

 //Function definitions  
 int readADC(unsigned int pin)  
 {  
      int fd;          //file pointer  
      char buf[MAX_BUF];     //file buffer  
      char val[4];     //holds up to 4 digits for ADC value  
      
      //Create the file path by concatenating the ADC pin number to the end of the string  
      //Stores the file path name string into "buf"  
      snprintf(buf, sizeof(buf), "/sys/devices/ocp.2/helper.14/AIN%d", pin);     //Concatenate ADC file name  
      
      fd = open(buf, O_RDONLY);     //open ADC as read only  
      
      //Will trigger if the ADC is not enabled  
      if (fd < 0) {  
           perror("ADC - problem opening ADC");  
      }//end if  
      
      read(fd, &val, 4);     //read ADC ing val (up to 4 digits 0-1799)  
      close(fd);     //close file and stop reading  
      
      return atoi(val);     //returns an integer value (rather than ascii)  
 }//end read ADC()

To compile, copy the text to a file and save the file as ADC_Example.c.   Then in the Terminal type:

 gcc -Wall ADC_Example.c -o ADC_Example.o  

Then run the program with:

 ./ADC_Example.o