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AR# 44205

DSP48 Slice - How do I connect up the PCIN when it is not connected to the PCOUT of an adjacent DSP48 slice?


How do I connect up the PCIN when it is not connected to the PCOUT of an adjacent DSP48 slice? Why do I see "noise" on the PCIN input?


The PCIN is a dedicated route to the PCOUT of an adjacent DSP48 slice.

In the various product "DSP48" user guides, Xilinx recommends that you tie the PCIN to '0's. Because the port must connect to a value, the device uses a value of all '0's to indicate to the tools that this port is not connected to an adjacent DSP48 slice. If the port is connected to anything other than '0's or the PCOUT of an adjacent DSP slice, then the implementation tools should issue an error, because these dedicated routes cannot be turned on or off.

The PCIN input is selected via the ZMUX inside of the DSP48 Slice. If you select the PCIN, it always gets the output of the adjacent DSP48A1 because of the dedicated routing. The illustration that follows is a screen shot from FGPA Editor. In this screen shot, the green lines at the top and bottom of the DSP48 slice show the dedicated route. If you look closely, you can see that they never pass through a PIP, or switch box, and this is because in the hardware they are always connected to the adjacent DSP slice.

Also, be aware that simulation treats each DSP48 slice independently and it does not know what the adjacent DSP slice will look like. If the inputs are all tied to all '0's, the simulation makes it appear as though a '0' is selected, but when you look at the hardware, the results do not match, and it might appear that there is "noise" on the PCIN bus. This is because what is really happening in hardware is that the PCOUT of the adjacent DSP slice is being selected. If a '0' is needed, use the OPMODE that selected the dedicated '0' input to the ZMUX.

For more information on a particular DSP48 slice, please see the appropriate product DSP48 slice device user guide:
AR# 44205
Date 09/23/2011
Status Active
Type General Article
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