Why is timer object firing at an incorrect frame rate?

I am using a timer object to try to enforce a frame rate of 25 Hz (0.04 seconds per period) for a few minutes. When I use tic/toc to see how long each 25-frame "second" takes, it is not one second. The time-per-second depends on what I have running in the TimerFcn. I have seen a string of 1.1-second "seconds", and other times there are strings of 0.92-second "seconds".

I am confused because when I use the "AveragePeriod" method, the timer always thinks it has perfect timing, and has an average period of 0.04 seconds. Even if the actual periods seem to be 10% off. Does the timer use a completely different system than the tic/toc functions?

I am running it in 'FixedRate' mode, so it should run the TimerFcn every 0.04 seconds. I have tried it with both 'drop' and 'queue' options for the BusyAction.

[Doug Hull]

How long does the timerfcn take to run?

Please show the code for construction of the timer object.

[James Potter]

Here's how I'm defining the timer:

trialTimer = timer('ExecutionMode', 'FixedRate', 'Period', 0.04, 'TimerFcn', @RunOneFrame, 'TasksToExecute', 115*25, 'StartDelay', 0);

As an experiment, I made RunOneFrame contain almost nothing. In the Profile Viewer, timer.wait took up over 95% of the self time, so excessive computation time wasn't an issue. Using get(trialTimer, 'AveragePeriod) gave answers of 0.04 seconds, but the time measured with tic/toc (output every second) showed that 0.94 seconds was passing for each 25-frame set, instead of 1 second.

The computer is running MATLAB 2012a, with Windows XP operating system. It has 1 GB of RAM (it's a little old).

[James Potter]

Again, the main thing that confuses me is the difference between what the timer thinks its average period is, and what the actual average period is. I don't know what would cause these to be different.