Evolution of a Blog

This blog has evolved as I have as a maker. It starts at the beginning of my journey where I began to re-tread my tires in the useful lore of micro electronics and the open-source software that can drive them. While building solutions around micro-electronics are still an occasional topic my more recent focus has been on the 3D Printing side of making.

Tuesday, October 1, 2013

Wiring the BBB for "The App"

Wiring the BBB for my application was pretty straight forward though the software side of the data collection proved to be a little less so.   


Integrating the Real Time Clock (RTC)

  • The BeagleBone Black, like other single board computers, does not have an onboard clock with battery backup to preserve current date and time.  
  • Theory of the case being that if the application needs one the date and time can be gotten from:
    • The network via NTP, or
    • An optional battery backed Real Time Clock.
  • Since ‘The App’ is not a network attached device the latter alternative has been implemented.
  • Luckily the BBB does come with the necessary hooks to easily integrate an RTC (assuming it is one of a list that is supported).
  • The RTC is connected using I2C and powered from the BBB as shown in the previous diagram.
  • Once it is connected the time on the BBB is set using NTP while it is connected to the network – ‘ntpdate -b -s -u pool.ntp.org’
  • Once the BBB is running the correct time it can be downloaded to the RTC – ‘hwclock -f /dev/rtc1 –w’
  • From that point the BBB can get the time from the RTC rather than the network – ‘hwclock -f /dev/rtc1 –s’
  • It is not essential that the BBB support a real time clock for operation of ‘The App’, however, doing so will allow files to be date and time stamped.

Integrating the Analog Digital Convertor (ADC)

  • The BBB has an onboard ADC, however, the entire ADC is allocated to the LCD cape!
  • Given this I have integrated an off board ADC, one that was designed for the Raspberry Pi, but that works with anything that communicates via I2C.
  • Wiring is shown by the preceding diagram and consists of five lines, two power, two for I2C, and a single line from our input source.
  • One of the disadvantages with the onboard ADC was it’s operating range for input (-1.8 to +1.8v).  The Pi ADC operates from 0-5v which better suits our requirement.
  • The bulk of the work of talking to an I2C device is done by a library installed via NPM – ‘npm install i2c’.  This library is available as part of Bonescript which ships with the BBB or as an install from npm per the above (which is what I am using since I don't have a need for the rest of Bonescript.
  • The code to access the ADC is shown below with some comments.
Initialize the interface library
var i2c = require('i2c');
Setup the address and command for the ADC.  The command translates to 12 bit resolution, gain of 1, in continuous mode
var address = 0x6A; var command = 0x90;
Use the library loaded above to instantiate a new wire (I2C) interface
var wire = new i2c(address,

    {device: '/dev/i2c-1', debug: false});
Setup an interval to capture observations.   5ms is as fast as we can go
intervalId = setInterval(function() {


}, 5);
Here is the callback for the above interval.  This function actually calls on the I2C interface to get four bytes of data from the ADC
function observeCallback() {

    wire.readBytes(command, 4, function(err, res) {



Within the observe function the two bytes of data that actually contain our reading are bit shifted into a measurement
function observe(buffer) {

    var voltageIn = (buffer[0] << 8) + buffer[1];

    voltageIn = (voltageIn / 2048) * 5.0;

There is a huge caveat on the above code.  Namely that when embedded within my application it will, at some point, cause a Segmentation Fault.  In a later post I will describe my solution to this rather fatal problem!

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