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from the doc lpmmult_accum is not a y = a*b+c operation, but it actually y = y + a*b, with y being an internal register within the function. Hence my question...
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You are right, y = y + a*b can't be implemented without a clock. Also the MegaWizard GUI doesn't have an option to omit the clock for the accumulator register. In the pdf user guide
integer arithmetic megafunctions, ALTMULT_ACCUM clock inputs are designated as optional. This is probably just a typo, or referring to a special case where accumulator feedback is disabled.
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