Hi all
I have been looking at the the butterfly_direct block in the FFT subsection
of the 10.1 library and have noticed a few things I hope someone can help me
with.
They relate to the logic that seems to be used to test for overflow ('of'
output). There are 4 Slice blocks extracting the most significant bits and
testing if they are all 1's (with an 'and' block) or all 0's (with a 'nor'
block). There are two things that seem to be out of place for me;
1. The blocks testing the real and imaginary components of each stream are
different. For the real component of a channel, all 4 Slices feed the 'nor'
block whereas, for the imaginary component, the 4th bit from MSB feeds the
'nor' block twice and the 3rd doesn't. I would guess that this is not meant
to be the case?
2. Taking the four MSBs means that (for twiddle_general_3_mult as twiddle
block at least) that these are all bits above the binary point i.e not
fractional bits. The test for all 1's would make this a small negative
number (i.e close to 0) and the test for all 0's would be a small positive
value, just above 0. Is this logic testing for overflow and underflow by
testing if the bits are all '1' or '0' respectively?
There is probably a clever answer to this or I have misunderstood something
somewhere. Could someone have a look at this and let me know what they
think.
Thanks
Andrew
