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Altera_Forum's avatar
Altera_Forum
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12 years ago

Mixed sign math problem

The background: I'm working on QAM modulation from a Cyclone II dev board with a DAC board I made connected to it. Works great as a DDS and everything other than the QAM math. The main QAM calcula...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
12 years ago

The problem is that a signed multiply effectively wastes one bit, which is necessary to avoid an overflow with -128*-128. To effectively utilize the result number range, dacfull must be resized to 15 bit with saturation logic applied, means 0x4000 (-16384) must be truncated to 0x3FFF (-16383).

ieee.fixed_pkg has a built-in saturation logic feature and is the most convient way to handle these problems. Or write your own functions to handle signed arithmetic with saturation.