Are there Simultaneous Switching Output (SSO) Guidelines for Spartan-3 devices?
Ground bounce can occur with high-speed digital integrated circuits when multiple outputs change states simultaneously, causing undesired transient behavior on an output or in the internal logic. This behavior is also referred to as the Simultaneous Switching Output (SSO) problem.
Ground bounce is primarily due to current changes in the combined inductance of ground pins, bond wires, and group metalization. The IC internal ground level deviates from the external system ground level for a short duration (a few nanoseconds) after multiple outputs change states simultaneously. Ground bounce affects stable low outputs and all inputs because they interpret the incoming signal by comparing it to the internal ground. If the ground bounce amplitude exceeds the actual instantaneous noise margin, then a non-changing input can be interpreted as changing.
The "DC and Switching Characteristics" section of the Spartan-3 data sheets, Table 21, provides the guidelines for the maximum number of simultaneously switching outputs allowed per output power/ground pair to avoid the effects of ground bounce. Refer to Table 22 for the number of effective output power/ground pairs for each Spartan-3 device and package combination. To access the Spartan-3 FPGA Family: DC and Switching Characteristics data sheet, go to: