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() {     observeCallback(); }, 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) {         observe(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!