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Scanning Srls Containing L2* Latches Without Losing Data

IP.com Disclosure Number: IPCOM000036615D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 62K

Publishing Venue

IBM

Related People

McAnney, WH: AUTHOR

Abstract

To reduce the overhead in implementing single-latch Level-Sensitive Scan Designs (LSSDs), a second data port can be added to the L2 latch (then called an L2* latch) so that both latches in a shift register latch (SRL) can be used as system latches [*]. Such an SRL is shown in Fig. 1.

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Scanning Srls Containing L2* Latches Without Losing Data

To reduce the overhead in implementing single-latch Level-Sensitive Scan Designs (LSSDs), a second data port can be added to the L2 latch (then called an L2* latch) so that both latches in a shift register latch (SRL) can be used as system latches [*]. Such an SRL is shown in Fig. 1.

However, using L2* latches makes it impossible to correctly read and restore the state of a machine. When data is shifted out of the SRLs, the data in either the L1 latches or the L2* latches will be lost, depending on which shift clock (A or
B) is turned on first. Similarly, an arbitrary state cannot be shifted in since two adjacent latches will always contain the same value.

(Image Omitted)

This article describes a method of reading and restoring the state of a machine with L2* SRLs that requires one SRL with a normal L2 latch (called a "nice" SRL) and two AND-Extend gates for each L2* SRL. Additionally, the disclosed method permits use of the scan path for system bring up and/or diagnosis.

L2* SRLs and "nice" SRLs are alternated on the shift register string, as shown in Fig. 2. (If there are more "nice" SRLs that are needed for this special shift register, the excess are formed into a separate shift register string.)

A two-input AND-Extend gate is added to each L2 and L2* latch of the special shift register. One leg of the gate is driven by the previous L2 or L2* latch in the shift register string. The other leg is driven by either a Shift A' clock if the gate is driving an L2* latch or a Shift B' if it is driving an L2...