A Proposal for Routing-Based Timing-Driven Scan Chain Ordering Puneet Gupta 1 Andrew B. Kahng 1 Stefanus Mantik 2 1: UC San Diego 2: Cadence Design Systems Inc.

Outline Introduction and Previous Work Motivation Our Contributions Timing Aware Connection Experiments Conclusions

Introduction and Previous Work Scan chains are commonly used to enhance testability. All flip-flops are chained to form a shift register. A goal of DFT is to minimize the impact of test circuitry on performance. Minimizing wirelength overhead of scan is essential. SI Q FF A SI Q FF B SI Q FF C PI PO

Introduction and Previous Work 3 different distance metrics considered for modeling scan chain ordering as TSP Nature of TSP –Cell-to-cell: Metric, symmetric –Pin-to-pin: Almost symmetric and almost metric –Pin-to-net: Asymmetric and Non- metric Our [ASPDAC’03] work: Trial routing driven pin-to-net distance metric gives best WL results (upto 80% better than industrial P&R tools) Cell-to-cell distance from FF B to FF A QQ SI A B Q Q A’ B’ Pin-to-pin distance QQ SI A” B” Pin-to-net distance

Routing Aware Scan Chain Ordering (ASPDAC’03) Incremental routing cost based on existing or anticipated routing. Considers both Q and Q’ outputs for the minimum wirelength connection. Driven by global routing or trial detailed routing. Q’ FF A SI FF B Q d(Q,SI) d(Q’,SI) Routing tree of Q’

Motivation No timing awareness in previously published literature Industry design methodologies constrain the FF output pin to be used for scan connection –60% of scan nets fall into this category in our test cases –Potentially large WL overhead –Unnecessarily constrains synthesis and layout

Our Contributions A method to compute timing driven incremental connection to existing route A buffer insertion method when timing is not met We highlight a potential use in scan chain ordering

Timing Aware Connection Divide routing tree into optimization segments. E.g., o 1 (begin 1  end 1 ) 1: influenced sink for o 1 2: uninfluenced sink Elmore delay model used

Optimal Attachment Point For uninfluenced sink 2  Linear dependence –r(l(root,b 21 )) * (c(x 1 +l(SI,v 1 ))+C SI )  Slack 1 –x min = Slack 1 / rc(l(root,b 21 )) – C SI /c - l(SI,v 1 ) For influenced sink 1  Quadratic dependence –r(l(root;b 11 )+l(begin 1 ;v 1 ) (c(x 1 +l(SI;v 1 ))+C SI )  Slack 2 –Compute x min using quadratic expression theory –Closed form solution for x min given in the paper Compute x min for all sinks and optimization segments Smallest x min  Minimum WL timing-feasible attachment point d(Q,SI)

ATSP Optimization Calculate optimal attachment points for (Q,SI) and (,SI) –TSP edge cost = min(d(Q,SI), d (,SI) ) No timing-feasible connection  label cost as a large number M Solve the ATSP –M cost edges marked for buffer insertion – ScanOpt used as the solver (

Buffer Insertion Assume fixed buffer sites with given locations Compute optimal attachment points for every (buffer site, M cost edge in the TSP tour) pair Cost of assigning a buffer site B to an edge e(ff1,ff2) = WL(ff1,B) + WL(B,ff2) Solution of the assignment problem  buffered scan chain solution

Tour Structure Dissimilarity Flows – I: Pin-to-pin distances –II: Pin-to-net –III: Timing Driven Testcases –1226 scan FFs –A, A swap, A expand have different placements Dissimilarity = % of different edges in the tour

Implementation with Industry Router Fails Vpin based incremental routing with Cadence Wroute: –Placing vpins on routing grid avoiding other pins –Placing vpins as regions as well as points –Routing from scratch as well as ECO –Pre-routing scan nets as well as routing all nets together None of the above work –Routing never completes

Conclusions We have given a proposal for –Incremental connection of a pin to a routing tree –Insertion of buffers in case timing is violated Basis for true timing driven scan chain ordering –Not able to validate the flow due to router not being able to handle vpins properly Need layout tools that can handle constraint dominated usage contexts