is there a way to hook up multiple VSSA pads

Question

< Tim Edwards> is there a way to hook up multiple VSSA pads to the same ground same PSUB on chip without LVS throwing a fit

Luis Henrique Rodovalho · Answer

You must insert a metal resistor between the nets This way you can short them You can make them by selecting rmetal layers

Jon Ho · Answer

< Luis Henrique Rodovalho> thanks for the help Just to clarify what does this look like in layout I don t imagine I should be doing anything in layout to actually short the nets together When you say rmetal layers does this mean I have to somehow connect two VSSA pads with this rmetal layer in layout Thanks in advance

Luis Henrique Rodovalho · Answer

You re only replacing a piece of the metal track with an resistor A RC extracted netlist would contain those resistors between the pads anyway In commercial software it is used a alias which in the end will be placed in the layout as a tiny resistor to separate the nets

Tim Edwards · Answer

< Jon Ho> The trick is to use the `isosub` layer in magic That will define a ground region that does not connect electrically to the rest of the substrate

Tim Edwards · Answer

I think I am interpreting your question correctly You are connecting e g `vssa1` and `vssa2` into your circuit but LVS complains that they are all shorted together and presumably also shorted to `vssd` which I think is the default substrate The two choices are to put `isosub` under all areas with devices connected to the same net or to put a deep nwell structure under all devices connected to the same net If you really want them isolated then the deep nwell structure is preferred

Jon Ho · Answer

Thank you guys for the help

Jon Ho · Answer

< Tim Edwards> do I have to use VSSD

Jon Ho · Answer

Currently I m only taking advantage of VSSA1 and VSSA2 for my ground pins since they re the closest pins to my design on the chip

Tim Edwards · Answer

You don t use vssd at all but LVS at the top level thinks everything has vssd as the substrate including the whole user project area I m still trying to work out issues related to that It s not an easy problem to solve and keep the user project area flexible

Mitch Bailey · Answer

< Tim Edwards> would a `subcut` box around the entire `user project wrapper` block at the `caravel core` level work I wonder how `user project wrapper` blocks with multiple substrate connections and multiple internal `subcut` layers would work in that case

Tim Edwards · Answer

< Mitch Bailey> They wouldn t which is why I m still thinking about it The best I ve come up with is to put a substrate cut under the user project if and only if the user project has no deep nwell or substrate cut areas itself But maybe there s a better solution