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Reading L2* and Gated B Clock Shift Register Latches Using a Bring-Up Tool to Scan Out the Values While Preserving Machine State

IP.com Disclosure Number: IPCOM000039914D
Original Publication Date: 1987-Aug-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Sotolongo, H: AUTHOR

Abstract

This article describes a technique for reading L2* and gated B clock shift register latches using a bring-up tool to scan the values without destroying the machine state. Fig. 1 is a representation of a shift register latch (SRL). It consists of two transparent latches interconnected as shown. The L1 latch has two sets of data and clock ports. The D and C inputs are the data and clock inputs for normal functional operation of the latch. The I and A inputs are the data and clock inputs used in scan mode. The B input is the clock input for the L2 latch. The outputs labeled L1+, L2+, etc., are the positive and negative polarity outputs for the respective latches. Transparent latch means that when the clock is active (in this case high level by definition) the output will follow the data input for that latch.

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Reading L2* and Gated B Clock Shift Register Latches Using a Bring-Up Tool to Scan Out the Values While Preserving Machine State

This article describes a technique for reading L2* and gated B clock shift register latches using a bring-up tool to scan the values without destroying the machine state. Fig. 1 is a representation of a shift register latch (SRL). It consists of two transparent latches interconnected as shown. The L1 latch has two sets of data and clock ports. The D and C inputs are the data and clock inputs for normal functional operation of the latch. The I and A inputs are the data and clock inputs used in scan mode. The B input is the clock input for the L2 latch. The outputs labeled L1+, L2+, etc., are the positive and negative polarity outputs for the respective latches. Transparent latch means that when the clock is active (in this case high level by definition) the output will follow the data input for that latch. The data will become latched and not change irrespective of the data input shortly after the clock goes inactive. In a level sensitive scan design (LSSD) the SRLs are strung together in a shift register ring, where the output of one L2 latch is tied to the I input of another SRL. This is repeated until all SRLs

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used in the VLSI chip are included in the ring. At least one input pin is used to feed the I input of the first SRL. The A and B clocks can be activated in an alternate fashion from external IO pins to either scan new data into the ring using the scan input, or to sample the data in the ring using the scan output. This is what is referred to as scanning mode. Fig. 2 shows what is referred to as an L2* SRL. This SRL has an additional set of data and clock inputs on the L2 latch. These inputs are typically used in functional mode applications, so it offers better utilization of the latches for operating functions. A latch with a gated B clock is any latch for which the B clock is not a free-running clock, but rather it is the ANDing of clock(s) and a control signal. Under these conditions the data in the L2 latch is not always the same as that of the L1 latch, since there may be periods when the data in the L2 latch is being retained, while the L1 latch is clocked with different data. In a VLSI design a common problem is not having enough visibility of the hardware for debugging design problems on the initial bring-up. It has been proposed on a LSSD to use the scan rings, used primarily to test the chips after manufacture, to provide visibility of the output of the latches in the chip during bring-up, by having an external bring-up tool (BUT) scan out the output of the latches and make this data accessible to the bring-up personnel for inspection. It also has been proposed that once the data is read, the BUT should be capable of

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restoring the state of the machine just before the read, by scanning in the data that was read previously. This is useful when tracking down...