While working on obstacle avoidance code for the autonomous mode, it became desirable to have a way to detect the amount of time that has expired. This is especially useful for detecting how long the robot has been stopped and how long it has been walking in a given direction.
Adding a timer interrupt in Basic Atom Pro code is actually quite easy - there is an example in the Basic Atom Pro manual.
The following two commands configure a timer interrupt on the Basic Atom Pro:
ONINTERRUPT TIMERAINT,handle_timerA
TMA= (TMA & 0xF0) 0x4 ;sets timera to increment once every 256 clock cycles
Here is the code that will run each time the interrupt is triggered:
handle_timerA
currenttimerval = currenttimerval + 1
resume
In the code listed above, TIMERAINT is an overflow interrupt and when this interrupt occurs, handle_timerA routine is executed and returned from by the resume command - the routine just counts 256 clock cycle clumps as one interval called currenttimerval.
Here is the code that starts the timer timing:
ENABLE TIMERAINT
Simple stuff. Stopping the timer is just as easy:
DISABLE TIMERAINT
So, we now have an easy to use timer that we just enable to start it ticking and disable to stop it. By the way, there is no explanation in the Basic Atom Pro manual regarding TMA other than a comment in a code listing. For an explanation on TMA, it is necessary to look in the h83664 manual. At first glance, it was a little challenging to understand but as with many things it was just a matter of staring at it long enough until the picture became clear, so to speak. There is also a comprehensive tutorial on the Lynxmotion forums that helped.
It wasn't until I stumbled upon the interrupts tuturial posted by a user in the Lynxmotion forums at http://www.lynxmotion.net/viewtopic.php?t=3496 that I found a caveat about using interrupts on the Basic Atom Pro. It seems that serout commands on the Basic Atom Pro are not always reliable when using interrupts. I confirmed this to be true after watching the Midnight Prowlerbot spasm and twitch in a random fit instead of walking as programmed. At first, I thought that maybe I had done something wrong in my code. After putting DISABLE TIMERAINT before every serout command and ENABLE TIMERAINT after each serout command, I found that everything worked fine. The robot walked as expected. The timer worked.
Unfortunately, enabling and disabling the timer interrupt before each serout command really affects accuracy of the timing. For instance, if the robot is instructed to perform a command that isn't executed each loop, such as an attack sequence, then the amount of time that the timer has been disabled has been changed when compared to just a walking sequence. Although this is not a crucial problem for things such as telling the robot to stop for a few seconds, it could be a problem if more accurate timing was needed.
Since I found the enable/disable interrupts before and after each serout to be an annoyance, I decided that it would be best to simply not use the interrupt. Instead, the Midnight Prowlerbot now counts the number of steps that it has taken when attempting to avoid an obstacle. For instance, this allows the robot to walk left three steps when any forward obstacle is detected instead of attempting to walk left for two seconds. In the future, more complex avoidance sequences than the left turn algorithm will be attempted.
