Nuts and Volts - January 2015

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calibration variable values for the potentiometer angle. We recorded the ADC value when the pot was pointed at zero, and recorded it again when pointed to 180. Based on those readings, we would then map the ADC values to angles between 0 and 180 degrees, and then store these values in volatile memory ('evaporates' when power is removed). Each time we re-powered the Arduino, we had to calibrate it again.

Conversions between these data types then become some relatively simple computing operations.

To keep things even simpler, we will use a special library (storeEEPROM) that automatically stores and retrieves 16-bit ints and 32-bit long ints in EEPROM. You can find this library in the a101_ch12_supplemental.zip file at the article link.

We somewhat alleviated that problem in Chapter 9 when we learned to use the Arduino with a battery that continued to provide power to the volatile memory after the USB connection was removed, but batteries get unplugged or eventually run out of juice. So, how do we get the Arduino to remember things when the power goes off? We use the EEPROM (Electrically Erasable If you want to see how these algorithms work, you can look at the original source code for each function.

The functions in that library are:

void storeIntEEPROM(int myAddress, int myInt)
int retrieveIntEEPROM (int myAddress)
void storeLongEEPROM (int myAddress,
 long int myInt)
long int retrieveLongEEPROM (int myAddress)

Programmable Read Only Memory) which is a form of non-volatile memory (doesn't 'evaporate' when the power is removed) that remembers the data until it is reprogrammed with new data. The Arduino UNO has

1,024 bytes of EEPROM. This memory is incredibly simple to use, plus we are provided with a library of functions that let us write or read bytes to addresses.

By now, you should be able to read each function description above and see what it does and how it is used. For instance, the first function stores a 16-bit int, myInt at the EEPROM location myAddress. The second function will retrieve a 16-bit int from the myAddress location. We will install and test this library in Lab 2.

Keeping a Timestamp for Data Sensing Charting Arduino Data on a PC

Well that's great, if you want to store eight-bit byte variables (numbers between 0 and 255). Unfortunately, there is a slight complication if you want to store other types of data that we've been using. The values from the ADC are 10-bit and may be from 0 to 1023. However, we store these values in 16-bit integers. As we saw in Chapter 9, Unix timestamps are 32-bit long integer values. We need an algorithm for converting these data types into bytes for storage and then retrieving those bytes and restoring the original data type. Since the ADC values are stored in 16-bit ints, we can convert them into two eight-bit bytes for storage. For the time value, we can convert and store 32-bit long integers as four eight-bit bytes.

Now that we know how to store raw data in an EEPROM, let's look at how we may show that data. We might write a program that reads four sensors once per second, and stores a data byte for each sensor in the EEPROM in blocks of four bytes each second. When we have finished reading that data after say, a minute, we may then send that data to a PC for interpretation. This sort of data is shown here with raw comma separated data:

0,152,252,89,2,165,249,77,5,177,244,66,10,188,239
,55,15,199,232,45,22,209,225,36,29,218,216,27,38,
227,206,20,48,234,196,13,58,241,185,8,69,246,173,
4,81,250,161,1,93,253,149,0,105,254,136,0,118,254
,123,1,131,253,111,3,143,251,98,6,156,248,86,11,1
68,243,74,17,180,237,63,24,191,230,52,32,202,222,
42,41,212,213,33,51,221,203,25,61,229,193,18,73,2
36,181,12,84,242,170,7,97,247,158,3,109,251,145,1
,122,253,132,0,134,254,120,0,147,254,107,1,160,25
3,94,4,172,250,82,8,183,246,71,13,195,241,59,19,2
05,235,49,26,215,228,39,34,224,220,31,44,231,210,
23,54,238,200,16,64,244,190,10,76,248,178,6,88,25
2,166,2,100,254,154,0,113,254,141,0,125,254,129,0
,138,252,116,2,151,249,103,5,163,245,91,9,175,240
,79,14,187,233,67,21,198,225,56,28,208,217,46,37,
217,207,37,47,226,197,28,57,233,186,21,68,240,175
,14,79,245,163,9,91,249,150,5,104,252,138,2,

When we list that data, we see that it doesn't make much sense to us. What is actually going on with these four sensors? We can put this data in a spreadsheet and have a chart drawn for the data showing the output of each of the four sensors as shown in Figure 1. Now, the data makes sense to us; we are seeing data that represents four sine waves.