Hello,
in the "XtremeDSP for Virtex-4 FPGAs User Guide", on Page 56 are in Table 1-13 some interesting functions mentioned, like AND and XOR and so on. Does anyone know how these functions are accomplished? Is there any documentation available which describes other more or less hidden (secret?) features of the DSP48 block?
Many thanks! Udo
AND function: result is 1 when all the input bits are 1, otherwise the result is 0. So if you add 1 to the input, what will you get (hint: carry out)?
XOR: a = 0, b = 0, result = 0 a = 0, b = 1, result = 1 a = 1, b = 0, result = 1 a = 1, b = 1, result = 0
if you add a and b, you get
a = 0, b = 0, result = 00 a = 0, b = 1, result = 01 a = 1, b = 0, result = 01 a = 1, b = 1, result = 10
result(0) is what you need.
HTH, Jim
These functions are accomplished by a "trick". The hardware can only perform mathematical operation, with carry. But by grounding every odd input, and ignoring every odd output, the carry does not propagate. Essentially, this divides the adder/subtractor into "watertight one-bit compartments". Peter Alfke, Xilinx Applications
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