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Permalink Reply by Bot Thoughts on July 20, 2012 at 10:27am

Your best bet is to think hard about what is happening and when.

Use paper and pencil to figure it out, cycle by cycle through the loop until you get the pattern of what is happening.

You say the GPS was refreshing more than once per second. Are you sure it wasn't just processing your print statement every time it read a single character? ;) Perhaps you should've scheduled a print/update task to run every 200-1000 ms ?

Again, write down, think through, fully understand what is happening and when. Figure out precisely when you want things to happen, then determine how to code it to do that.

You say refresh rate is excellent. What's that based on, serial output? Ok that's fine for a human reading the info. But that doesn't guarantee the timings are right for calculating anything.  Is it running the code on a proper schedule? It has to if you want to have any hope of correctly calculating time-dependent variables, like calculating heading from heading rate (gyro).

The expression (now%100)>0 means that it will run anytime millis() isn't a multiple of 100. So it will run at least once every millisecond except at 100, 200, 300, 400, ...  In other words 197, 198, 199, 201, 202, ... 298, 299, 301, 

Is that really what you wanted?

If not, spend time understanding how modulo works. Write test code on a more accessible platform; like C, Perl, Python, etc., on your PC.

If you want a task to run every 100ms there's a few ways to do it. Modulus (remainder) is what I've used in the past.

I used the expression if ((timeNow % 100) == 0) because it evaluates true at multiples of 100. If you want to run something every 100ms but at say 150, 250, 350, 450...  use ((timeNow % 100) == 50)

I wrote a perl script to demonstrate this.

http://pastebin.com/ejjpK6Ew

You can use that to play around more to better understand modulo and scheduling tasks.

Meanwhile here's a quiz :D Let's say I have something like this:

void loop () {

int timeNow = millis();

if ((timeNow%10) == 0) do1();

if ((timeNow%20) == 0) do3();

}

How often will do1() and do2() run if...

• do1() takes 20 milliseconds to execute and do2() takes 1ms to operate?
• do1() takes 1ms and do2() takes 20ms?
• do1() and do2() take 0.1 millisecond to run?

Hm, I think I'm going to have to write a blog article about this topic...

Permalink Reply by Brett Smith on July 21, 2012 at 10:35pm

I timed all of my functions to see how long they would take to run. I was almost ready to set up the duty cycles like you said, when I ran into a new problem!
I know I am getting a little off topic here, but I need help...
I tried mounting my sensors to the plane to make sure I would not have issues with electronic interference. As it turns out, I am having mechanical interference. When I hold the sensors above the plane, no problems. As soon as the sensor make contact with the plane with the motor running, my sensor values go all over the place. What should I do!!!
I am leaving for a trip to Idaho next weekend, where I was planning the maiden flight of my system. If need be I can order new sensors to be delivered to our property...
I am using some osepp brand sensor I bought from frys. They run an ADXL345 (accelerometer) and MPU 3050(gyroscope).
I wrote a program to try and filter the values. I filled an array of the 20 previous values and took the median. This worked on an earlier project but is no longer sufficient. In addition, it creates a sizable delay in my sensor values...
What should I do?

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