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0 pointskccqzy1y ago0 comments

I have worked on firmware that has plenty of fixed point arithmetic. The firmware usually runs on processors without hardware floating point units. For example certain Tesla ECUs use 32-bit integers where they divide it into four bits of integer part and 28 bits of fractional part. So values are scaled by 2^28.

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phkahler1y ago

>> The firmware usually runs on processors without hardware floating point units.

I'm working on control code one an ARM cortex-M4f. I wrote it all in fixed point because I don't trust an FPU to be faster, and I also like to have a 32bit accumulator instead of 24bit. I recently converted it all to floating point since we have the M4f part (f indicate FPU), and it's a little slower now. I did get to remove some limit checking since I can rely on the calculations being inside the limits but it's still a little slower than my fixed point implementation.

sitkack1y ago

The other great thing about going fixed point is that it doesn't expose you to device specific floating point bugs, making your embedded code way more portable and easier to test.

32b float on your embedded device doesn't necessary match your 32b float running on your dev machine.

bobmcnamara1y ago

32b float can match your desktop. Really just takes a few compiler flags(like avoiding -funsafe-math), setting rounding modes, and not using the 80bit Intel mode(largely disused after 64bit transition).

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cpeterso1y ago

Why did you decide to convert your code to floating point if your fixed point implementation was faster and already written?

gatane1y ago

Are there any good benchmarks for float vs fixed point, specially for ARM systems?

evoke49081y ago

Just look at the instruction set for your particular CPU. Every CPU is different, but in most architectures I've seen, floating point operations are 2-3 times slower for the same word size.

Single float adds are usually 2 or 3 CPU cycles while single-word integer adds are usually 1 cycle.

Again, this is extremely dependent on the particular CPU you have. Some architectures do have single-cycle FPU operations, but it's not very common in microcontrollers as far as I can tell.

EasyMark1y ago

That would vary wildly with the ARM chip you are talking about. I would say figure out which ARM you’re interested in and go down the rabbit hole from there.

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