Question

I’m reading values from a sensor via serial (accelerometer values), in a loop similar to this:

while( 1 ) {
   vector values = getAccelerometerValues();
   // Calculate velocity
   // Calculate total displacement
   if ( displacement == 0 )
       print("Back at origin");
}

I know the time that it takes per sample, which is taken care of in the getAccelerometerValues(), so I have a time-period to calculate velocity, displacement etc. I sample at approximately 120 samples / second.

This works, but there are bugs (non-precise accelerometer values, floating-point errors, etc), and calibrating and compensating to get reasonably accurate displacement values is proving difficult.

I’m having great amounts of trouble finding a process to debug the loop. If I use a debugger (my code happens to be written in C++, and I’m slowly learning to use gdb rather than print statements), I have issues stepping through and pushing my sensor around to get an accelerometer reading at the point in time that the debugger executes the line. That is, it’s very difficult to get the timing of “continue to next line” and “pushing sensor so it’s accelerating” right.

I can use lots of print statements, which tend to fly past on-screen, but due to the number of samples, this gets tedious and difficult to derive where the problems are, particularly if there’s more than one print statement per loop tick.

I can decrease the number of samples, which improves the readability of the programs output, but drastically decreases the reliability of the acceleration values I poll from the sensor; if I poll at 1Hz, the chances of polling the accelerometer value while it’s undergoing acceleration drop considerably.

Overall, I’m having issues stepping through the code and using realistic data; I can step through it with non-useful data, or I can not really step through it with better data.

I’m assuming print statements aren’t the best debugging method for this scenario. What’s a better option? Are there any kinds of resources that I would find useful (am I missing something with gdb, or are there other tools that I could use)? I’m struggling to develop a methodology to debug this.

Answer 1

A sensible approach would be to use some interface for getAccelerometerValues() that you can substitute at either runtime or build-time, such as passing in a base-class pointer with a virtual method to override in the concrete derived class.

I can describe the mechanism in more detail if you need, but the ideal is to be able to run the same loop against:

• real live data
• real live data (and save it to file)
• canned real data saved from a previous run
• fake data you cooked up as a test case

Note in particular that the ‘replay’ version(s) should be easy to debug, if each call just returns the next data from the file.