Description
On some devices, including APEXTM 20K, APEX 20KE, FLEX® 10K, FLEX 10KE, FLEX 8000, FLEX 6000, MAX® 7000AE, and MAX 7000B devices, Altera offers a feature that allows I/O pins to be connected directly to the devices' internal ground. This feature enhances the noise immunity of the device, even further. This technique is particularly useful where ground bounce is a design consideration. For example, if there is an important output pin that is used as a clock for another device, that signal can be shielded from ground-bounce effects by using the two adjacent pins as programmable grounds.
In addition, the QuartusTM and MAX PLUS® II software will always use the programmable ground feature on I/O pins that are not used in a design. These pins are designated as GND in the text-based Report File (.rpt) or the compiler report window. By doing so, the Quartus and MAX PLUS II software provides the best possible noise immunity for each device application.
Using programmable grounds does not require any logic resources in the Altera® device. Also, other MAX family devices can emulate programmable ground pins by using the macrocell to drive out GND on any unused I/O pin.