Schottky Ttl 1970s Confront Ground Bounce and Switching Noise
# Schottky TTL (early 1970s): Step 14 — Confront Ground-Bounce and Switching Noise
Confront Ground-Bounce and Switching Noise. Ground bounce arises when several output transistors on the same package switch simultaneously, each briefly drawing a transient current spike through the shared lead inductance of the package's ground pin and bond wire; that inductance, however small, develops a voltage (V = L di/dt) across itself during the fast current transient, momentarily lifting the chip's internal ground reference away from the board's ground plane.
## 1. Where Ground Bounce Comes From
Ground bounce arises when several output transistors on the same package switch simultaneously, each briefly drawing a transient current spike through the shared lead inductance of the package's ground pin and bond wire; that inductance, however small, develops a voltage (V = L di/dt) across itself during the fast current transient, momentarily lifting the chip's internal ground reference away from the board's ground plane. Because Schottky TTL's faster edge rates (74S especially) increase di/dt relative to standard TTL for a given load, ground bounce amplitude scales up correspondingly, and on packages with many outputs switching together (wide counters, bus drivers, registers), the bounce could become large enough to transiently violate the receiving gate's input threshold on an unrelated, supposedly-quiet signal sharing the same ground return.
## 2. Real Diagram: Ground-Bounce Waveform on a Multi-Output Package
## 3. Mitigations Carried into Board and Package Design
Board and system designers addressed ground bounce through a combination of techniques that became standard practice through the Schottky TTL and later 74ALS/74F era: generous decoupling capacitance placed close to each package to supply the transient current locally rather than through long power-plane traces, multiple ground pins on wide packages to divide the shared lead inductance across several lower-inductance paths, and staggered-output design on bus-driver parts to avoid every output transitioning at exactly the same instant. These mitigations, developed under pressure from Schottky TTL's faster edges, carried forward essentially unchanged into the CMOS glue-logic era discussed in Step 18, since ground bounce is a package/board phenomenon, not a transistor-family-specific one.
## 4. Companion Diagram: Stage Sequence and Trend Summary
## Confront Ground-Bounce and Switching Noise's Place in the Process Lineage
This is step 14 of 20 in the Schottky TTL (early 1970s) series, examining the ground-bounce consequence of Schottky TTL's faster switching edges introduced in Step 13.