Answers Database VIEWSIM timing simulation: "x" on the output of flip flop. XSIMMAKE Record #486 Product Family: Software Product Line: ViewLogic Product Part: viewsim Product Version: 5.3 Problem Title: VIEWSIM timing simulation: "x" on the output of flip flop. XSIMMAKE Problem Description: Viewsim may show an "X" on the output of a DFF which XBLOX has merged into the IOB during timing simulation. If your design has been run through XBLOX and the archopt switch has not been set to false, or if the attribute STYLE=IOB, XBLOX will merge DFFs into IOBs where it can. If you try forcing a value on the node that is shown in the schematic as being connected to the D-input of the DFF and then clock it, the value that appears on the output of the DFF may not necessarily be the value you expect. Most likely it will have the value "X". XBLOX merges a flip flop in the schematic to the net that connects the IPAD to the IBUF of the parti cular IOB where the flip flop was merged. For instance, given a PAD that is connected to an IBUF thr ough a net called "ibuf_in", and the output of the IBUF is connected to a flip flop through a net c alled "d_in". After XBLOX merges the flip flop into the IOB, the net connected to the input of the flip flop is called "ibuf_in". Therefore, forcing a value on the net called "d_in" is irrelevant to the flip flop. Assume for example that you are using an XBLOX INPUTS symbol instead of the IPAD and IBUF. Assume the label on the INPUTS is " data_in", and bit zero of the bus is connected to the FD. After XBLOX has merged the flip flop, the net connected to the input of the flip flop is called "data_in\pad0". Solution 1: Determine which flip flops in your design have been merged into IOBs by XBLOX. To force values at the d-inputs of the DFFs, apply the force at the input to the corresponding IBUF, not at the input to the flip flop as shown in the schematic. End of Record #486 - Last Modified: 11/15/96 18:15

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