Simultaneous Switching Outputs

UltraScale Architecture SelectIO Resources User Guide (UG571)

Document ID
UG571
Release Date
2022-08-31
Revision
1.14 English

Due to package inductance, each part/package supports a limited number of simultaneous switching outputs (SSOs), particularly when using fast, high-drive outputs. Fast, high-drive outputs should only be used when required by the application.

The SSN predictor tool provides a way of analyzing the amount of noise margin on each I/O pin in a design based on information for the pin (the victim), as well as all other pins (aggressors) in the design. The tool takes into account I/O pin locations, I/O standards, slew rates, and terminations used, and provides a value for the noise margin for each pin based on these characteristics. The noise margin does not include any system-level characteristics such as board trace cross-talk or reflections due to board impedance discontinuities.

Ground or power bounce occurs when a large number of outputs simultaneously switch in the same direction. The output drive transistors all conduct current to a common rail. Low-to-High transitions connect to the VCCO rail, while High-to-Low transitions connect to the GND rail. The resulting cumulative current transient induces a voltage difference across the inductance that exists between the internal and external ground levels, or internal and external VCCO levels. The inductance is associated with bumps, die routing, package routing, and ball inductance. Any SSO-induced voltage consequently affects internal switching noise margins and ultimately signal quality.

The SSN predictor results assume that the device is soldered on the PCB and that the board uses sound design practices. The noise margin values do not apply for devices mounted in sockets due to the additional BGA ball inductance introduced by the socket.