November 2009 Archives

This is the latest version of the ATMega168 version of the code which includes all of the new servo commands that I wrote about here including the multi-move command and the unit tests that I spoke of here. The controller allows you to set minimum, maximum and startup servo positions for each servo which can be saved into eeprom and used every time the controller is powered up. It also includes a programmable, per servo, "centre adjust" value which can be used to adjust for servos that are installed slightly off centre.

Whilst developing the servo controller I often wished I had an oscilloscope, being able to visualise the signals that I was generating would have been useful many times. Due to the cost, choice and the fact that I eventually worked out what I was doing wrong in each situation I am continuing to delay getting an oscilloscope but today I saw a cheap little device that could be useful as I tune the controller in future. My 'new products' feed from CoolComponents had the TextStar Serial LCD Display in it this morning. This is a neat little programmable display which has a 'servo signal display' mode which enables it to display the pulse length and refresh rate of a servo signal. Given the cheap price and the fact that I'm sure the LCD display in itself will be useful I ordered one. I hope to hook it up to my controller so that I can see what's being generated. The datasheet for the device is here.

I decided that putting 'printf' style debug output into the routine to attempt to debug it from my PC based control software was just the wrong way to go about finding the problems and so I set off on a mission to finally get some unit testing into my code. This worked out well and I now have over 80 tests for all of the serial command code. I found a few subtle bugs and I'm in a much better position for reactoring away some duplicate code and other design smells. I also feel much more confident about making the other wide ranging changes that I will eventually make when I switch to using interrupt driven serial comms and move to a 16bit control value for the PWM signals...

So now I have to write the tests for the code that I know has some bugs in it... The problem I have with the code under test is that it's quite big. In fact it's quite a bit bigger, and more complex, than all of the other serial protocol routines that I've tested. The initial tests are easy enough, parameter validation etc, but the main tests involve me testing a large block of code that appears to have at least one 'oh dear I've gone off into an infinite loop' bug in it somewhere under some input data conditions... At first I thought I would have to break the code down into smaller functions that were easier to test and then I realised that I can already test the code in smaller sections, just as long as I do so backwards.

The code is one long function that we jump into and which jumps back out to the command accumulation loop. Throughout the code there are various labels that break up the various sections of the code. If I structure my tests so that I test the code from the end back towards the front I can simply set up the environment with the data that each stage expects and then jump to a label that processes the data. So, if the code takes input data at A and then processes it via B, C, D, E and F, I can first test pushing the kind of data that E should produce by jumping to F. Once that's works I can test E with the data that D would produce, etc... Once I get to A the whole thing is tested...

We'll see how it goes...

Ah well, it's not a problem really... I'm now at 66 tests and 59% of my code space used up. I've got pretty complete coverage up to the "stop servos" command. I have the query commands to test and then I'm finally on to the multiple servo move command... Then I have the PWM code to test, but I expect I'll take a break from the testing for a while once I get the multiple servo move command to actually work!