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Controlled Enable of Off-Chip Drivers During Lssd Testing

IP.com Disclosure Number: IPCOM000038876D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Storey, TM: AUTHOR

Abstract

Disclosed is a method of separating changes in circuit current initiated by switching off-chip drivers from changes in circuit current which occur as a result of internal device switching. With the advent of VLSI, in which internal circuit elements are increasingly smaller and denser while the off-chip drivers (OCDs) must maintain reasonable current-carrying capabilities, noise which occurs during chip and module testing becomes a critical problem. In CMOS VLSI designs, a large current surge occurs as the OCDs change state. The effect of this current surge is enhanced by the unbypassed inductance of a chip/module tester. If the change in current or noise is not controlled, noise can cause dips or ground bounce in the power supply which may alter the state of internal latches.

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Controlled Enable of Off-Chip Drivers During Lssd Testing

Disclosed is a method of separating changes in circuit current initiated by switching off-chip drivers from changes in circuit current which occur as a result of internal device switching. With the advent of VLSI, in which internal circuit elements are increasingly smaller and denser while the off-chip drivers (OCDs) must maintain reasonable current-carrying capabilities, noise which occurs during chip and module testing becomes a critical problem. In CMOS VLSI designs, a large current surge occurs as the OCDs change state. The effect of this current surge is enhanced by the unbypassed inductance of a chip/module tester. If the change in current or noise is not controlled, noise can cause dips or ground bounce in the power supply which may alter the state of internal latches. The solution disclosed is to separate the current changes initiated by the switching of the OCDs from the current changes occurring as a result of internal device switching. The disclosed approach delays the enabling of the OCDs until switching of the internal logic has been completed, thereby reducing the current surge peaks and any associated noise. Referring to the figure, the solution includes the following steps: 1. Start of a cycle (t0): Apply input changes while

the OCDs are enabled.

2. At time t1, apply clock pulses associated with a

first SRL L1 (A and C Clocks).

3. At time t2, disable OCDs by placing them in a high

impedance...