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Dec 30, 2012, 03:10 AM
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Revenge of fixed point


In exchange for expanding your executable by a large chunk, the Arduino toolchain for ARM does compile most of the floating point operations for the STM32F4, but some trig functions don't work & USB doesn't work. It uses software floating point, of course. It would take some hunting for the right compiler flags for that one.

You'd be amazed what can be accomplished with software fixed point routines. All your favorite trig functions can be performed with lookup tables, in a reasonable amount of memory. Bringing up a fixed point navigation routine takes a lot less time than bringing up a floating point compiler toolchain & it's still going to be faster than software floating point.

There's basically execution time, development time, & longevity of the microprocessor. If your execution doesn't require the fastest floating point, all your work on the toolchain has no effect on the output. The average microprocessor is end of lifed after 1 year so all your investment in bringing it up has to be disposable. Bringing up the most advanced gcc optimizations for a new microprocessor can expand the development time weeks & months beyond implementing fixed point routines.

If another new, ground up, autopilot is written for ARM, it's going to be fixed point. More likely, the next outdoor vehicle would use Arducopter. An outdoor vehicle hasn't flown in over 1 year. The 1 vehicle would take a lot of debugging to fly again & it still uses a ground based autopilot.

Having used the ground based autopilot & a tri copter for 3 years, there's now a lot more interest in the faster feedback of an embedded autopilot on a quad copter & a tablet for a human interface. The tablet would need a radio breakout board to have enough range & reliability.

An autopilot isn't a massive software project. For consumer grade sensors & flight controls, the sensor data is noisy enough & the environment is unstable enough that the software doesn't need razor accurate double precision floating point math solutions. Most software accuracy is being outdone with better sensors & faster update rates.

Embedded autopilot would still be impractical for the indoor vehicles.
Last edited by Jack Crossfire; Dec 30, 2012 at 03:21 AM.
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