Quote:
Originally Posted by Greg McKaskle
The timer doesn't control the rate of the loop. It merely measures the time from one execution to the next. If you have code within teleop that takes more than 20ms to run, you will skip DS updates. I see this quite often on team DS logs, even teams from Einstein. It would be odd that the periodic is not a multiple of 20ms. And unless the network and/or DS are stressed out, I'd expect jitter numbers like Joe showed above.
I'm not sure how Java PID does its scheduling, and when teams spawn another thread and set its rate, I'm not sure what they are using to schedule.
Team 358's article is about timing using LabVIEW. LabVIEW has two different schedulers. One has ms level resolution and is used for general VI execution. It uses a pool of threads at various priorities, and the LV wait and wait ms functions and a plain while loop will operate in this scheduler. The TimedLoop uses its own scheduler that runs at an elevated priority and has advanced scheduling options -- quite advanced. It can schedule at sub millisecond periods and implements a variety of policies. Only Timed structure code runs in this scheduler.
The timed loop is also instrumented so that it is easy to determine start and stop times, missed deadlines, adjust priorities and other things. I'd highly recommend that you instrument your PID not only to measure its actual period, but also its execution time. You could also do this with a profiler if you have one. This will let you know the cost of the operation you are scheduling.
Greg McKaskle
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Yes, we understand that the timer is a free-running stop watch that we are using to instrument our loops. This is why in the OP we were concerned that the teleopPeriodic was reporting 50ms on a program that did nothing but report the time while running tethered. Switching to the FPGA timer now correctly reports a 20ms teleopPeriodic rate.
What we are trying to do is create another thread outside of teleopPeriodic for doing the PID calculations at a faster, more consistent rate. We believe we have accomplished this in JAVA. We were hoping to achieve a 10ms period but appear to be processor limited to 15ms. This seems like a long time given other teams are talking about 5ms loops (not sure if they measured actual loop times). We do have some calculations in our loop, but I would not have expected it to limit the period by so much. We are looking into the bottleneck but time on the robot is limited with bag & tag in a few days. Worst case, the performance with 15ms seems fine for competition.