Cordic trig assembly code for PIC18f

[ScaleRobotics]

Hi Chris,

Well, I might have to admit that the cordic may not be the best solution for a single axis tilt sensor. It is a natural for a dual or triple axis sensor. If you just have a single axis sensor, I might suggest a lookup table. Now I'm not really good at converting mathematical equations back and forth, so I can't say for sure if you are on the right track with the one you have. It doesn't resemble the ones I have used enough for me to tell. But I can show you some that I think would work. And, perhaps yours would work too. I just can't tell from here.

Here is the app note I worked with before. As you can see, the arctan equation "theta = arctan(Ax/Az)" on page 4 is a pretty cool thing. Of course, this one requires a two axis sensor, but it keeps resolution up to about 1.3 degrees. It also fits in pretty well with are atan2 cordic.
Attachment 4533

Now for a single axis sensor, your resolution at near 90 degree angles goes down to about 9 degree accuracy. This seems to lend itself better to just a lookup table. But, if you really wanted to use the cordic, you could.

For a single axis sensor, one equation for determining angle is arcsin (x) where x is acceleration in g's . Our cordic, can't do arcsin, but it can do arctan. We can use the following equation from Wikipedia:




and solve for theta.

atan(y) = atan2(y,1)

But, in our case, we are feeding y with a value which at its peak, = 1g. So, whatever value your sensor gives for 1g, you should fill in for the "1" value. y will equal 2 arctan x/((1+sqr(1-x^2)) , then run it through the cordic. I just feel if its going to be off by up to 9 degrees (because we are using a single sensor) why not just use a lookup table.