Spartan3 or 3E pins to GND

As outputs driven low, IOBs can be a reasonable ground. It's not a precise ground but similar to adding a low value resistor to your other PCB. If your other device is a low current device (I'm looking at using this scheme for

Yes, an active Low output represents a low impedance to ground, somewhere around 10 Ohm. You can easily measure this by pulling the pin to Vcc through a 100 Ohm resistor, and measuring the pin voltage.

Users have also used such an output in the opposite way: Connect the pin to the pc-board ground, and thus reduce the ground-bounce by providing additional connections between chip ground and pc-board ground. Peter Alfke, Xilinx

Yes, an active Low output represents a low impedance to ground, somewhere around 10 Ohm. You can easily measure this by pulling the pin to Vcc through a 100 Ohm resistor, and measuring the pin voltage.

Users have also used such an output in the opposite way: Connect the pin to the pc-board ground, and thus reduce the ground-bounce by providing additional connections between chip ground and pc-board ground. Peter Alfke, Xilinx

Peter; So this is what happens when a CPLD pin is set as a PGND? =20

-Dave Pollum