XPLA Designer v2.1 User's Manual

Presets are done in a similar fashion, except that a different dot extension is used (seeFigure 27).Resets and presets made directly from input signals and feedbacks can be either a sumterm or a product term; but they cannot be a sum of products. For example, supposethere is a design with four inputs A, B, C, and D. You could create a reset as a productterm likeA & B & C & Dor as a sum term likeA * B * C * D.By contrast, sums of products and products of sums likeor(A * B) + (C * !D)(!A # B) & (!C # !D)are not directly supported.Complicated reset and preset control term equations that consist of sums of product termsmust use a buried node with the ‘keep’ attribute. The node is assigned the sum ofproducts control term equation, and then the reset or preset is assigned to the node.Because of the ‘keep’ specification, the node will not be collapsed. The following 4 bitcounter example illustrates how to make a reset control term that is a sum of products. Inthis example, the counter is reset whenever signal rst is high or both of the most significantcounter bits are high.Module cntr4bitTitle '4 bit counter with sum of products reset'clkrstq3,q2,q1,q0n1pin;pin;pin istype 'reg_d,buffer';node istype 'keep';count = [q3,q2,q1,q0];equationscount.c = clk;52