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Like anything in the real world, your sensors and devices won't necessarily work perfectly all the time. Many analog semiconductor devices are light, temperature, and pressure sensitive, so your actual readings can vary depending on outside conditions. Also, while sensors are usually manufactured to within pretty tight tolerances, every sensor has some finite error that you will need to account for to get maximum performance.
Some kind of calibration routine can help alleviate many of these concerns. By calibrating your sensors against a trusted source, you can correct a lot of the skew caused by outside conditions. Calibration will also eliminate any steady state error caused by the device itself.
A calibration routine collects a set of known data points and scales the input signal to fit the calibration data. Since we are calibrating a fairly linear device, we only need two data points to determine its operating curve. To get the most out of the calibration, you should use two data points that are as far from each other as possible. In the case of our light sensor, that will be darkness, and a reference value that is very bright . You can either download a free smartphone LUX meter to measure a reference source to use to calibrate your board, or use an approximation from the computer monitors in the lab. The monitors are about 620 lux when showing white at full brightness with the measurement device held within a few inches of the monitor.
Calibration routine pseudocode (not in c syntax)
flag=0;
interrupt function: calibrate()returns: lowMeasured, highMeasured
{if flag=0:
set low and set flag=1if flag=1:
set highreturn
}
Code Composer Studio has a special way of handling interrupts in C. It uses "compiler directives" (special instructions to the compiler, assembler, and linker) to specify which functions should go in the interrupt vector table. ISRs in C are written like any other function, except that they can take and return no values. This fits with the convention that ISRs don't interfere with or depend on the code around them.
Formatting an interrupt in code composer c
#pragma vector=PORT3_VECTOR
//compiler directive saying that this function should correspond with the port3 interrupt vector__interrupt void interruptHandle()
{//your ISR CODE
}void main()
{... all setup
__enable_interrupt();//Enables general maskable interrupts
... the rest of your program}
Using Code Composer Studio 5, write a C language program to drive a simple 3 digit lux meter. Use the ADC to read the voltage and display back the actual lux value. Write a calibration routine for your lux meter and an output routine that will allow you to display each of the output digits.
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