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Altera_Forum's avatar
Altera_Forum
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11 years ago

sqrtf custom instruction gives sometimes absolutely wrong results

I use Quartus 13.1.0.182 web edition with a Cyclone III FPGA The project is compiled with the Floating point custom instruction 2.0 from Qsys. My code calculates the square root and some time...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
11 years ago

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Looks that way, probably failing to increment the exponent when the mantissa is rounded up.

FWIW you'll probably get better precision from calculating one of sqrt((1+sin)*(1-sin)) or sqrt(0.5 - sin*sin + 0.5).

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Thank you for your reply.

Even if your fix works in this special case (not already test), I don't really trust this sqrt hardware function! There is no guaranty it would not do the same with an other rounded. I should ask Altera directly