Simultaneous Switching Noise (Ssn)

Simultaneous Switching Noise (SSN) is the electrical noise generated when many I/O drivers or internal circuits switch at the same time, causing large transient currents through the parasitic inductance and resistance of power and ground paths. SSN is a superset of ground bounce that encompasses noise on both power (VDD) and ground (VSS) networks.

The Physics of SSN

• Each switching output draws a pulse of current from VDD (when switching low-to-high) or pushes current into VSS (when switching high-to-low).
• When $N$ outputs switch simultaneously, the aggregate current change is approximately $N \times dI/dt$.
• This current flows through the shared inductance ($L$) of the package and on-die power/ground networks, creating noise voltage: $V_{noise} = L \cdot N \cdot \frac{dI}{dt}$.

SSN Components

• Power Bounce (VDD Droop): When many outputs switch high, they draw current from VDD simultaneously → VDD droops below nominal.
• Ground Bounce (VSS Rise): When many outputs switch low, they push current through VSS → VSS rises above true ground.
• Combined Effect: The effective voltage swing seen by circuits is reduced: $V_{effective} = (VDD - droop) - (VSS + bounce)$.

Impact on Chip Performance

• I/O Signal Integrity: Non-switching outputs may glitch — a quiet LOW output referenced to a bounced ground appears HIGH.
• Core Logic Errors: Internal circuits referenced to noisy power rails may see reduced noise margins, causing setup/hold violations.
• Jitter on Clocks: SSN on clock distribution causes timing uncertainty.
• Analog Interference: ADC accuracy, PLL stability, and reference voltage quality all degrade with SSN.

SSN Analysis

• Worst-Case Pattern: Identify the switching pattern that maximizes simultaneous switching — typically all outputs in a bank switching in the same direction at the same clock edge.
• Package Model: Include accurate package parasitics — bond wire/bump inductance, plane capacitance, mutual inductance between adjacent pins.
• Frequency Domain: Analyze the power delivery network impedance — SSN is worst at frequencies where the PDN impedance is highest (typically near the package resonance, 100 MHz–1 GHz).

SSN Mitigation

• Reduce Simultaneous Switching: Stagger output enables, use multiple clock phases, limit the number of outputs per power/ground group.
• Increase Power/Ground Connections: More pins, bumps, or balls dedicated to power and ground — reduces shared inductance.
• On-Die Decoupling: Decaps supply local charge during switching transients.
• Controlled Slew Rate: Limit driver edge rates — slower transitions reduce $dI/dt$ at the cost of speed.
• Separated Power Domains: Isolate noisy I/O banks from quiet I/O and core logic.
• SSO Guidelines: Follow package SSO limits — the maximum number of simultaneously switching outputs per power/ground pair.

SSN is the combined power and ground noise challenge of modern IC design — managing it requires holistic co-design of the chip I/O, package, and power delivery network.