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An Impulse-Response Based Methodology for Modeling Complex Interconnect Networks Zeynep Dilli, Neil Goldsman, Akın Aktürk Dept. of Electrical and Computer Eng. University of Maryland, College Park ISDRS 2005

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Interconnect Network Modeling Objective: Investigate the response of a complex on-chip interconnect network to external RF interference or internal coupling between different chip regions Full-chip electromagnetic simulation: Too computationally- intensive Full-wave simulation possible for small “unit cell”s: Simple seed structures of single and coupled interconnects, combined to form the network. We have developed a methodology to solve for the response of such a network composed of unit cells with random inputs Sample unit cells for a two-metal process

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Interconnect Network Modeling On-chip interconnects on lossy substrates: capacitively and inductively coupled – Characterized with S-parameter measurements – Equivalent circuit models found by parameter-fitting For small interconnect unit cells, create an equivalent circuit model from EM simulation results/S-parameters

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Interconnect Network Modeling The interconnect network is a linear time invariant system: It is straightforward to calculate the output to any input distribution in space and time from the impulse responses.

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Response to a General Input from Impulse Responses [x-x i ] [ t] h i [x,t] f[x,t] We can write these input components f i [t] as Writing f i [t] as the sum of a series of time-impulses marching in time: [x-x i ]= 1, x=x i 0, else Define the unit impulse at point x i : We calculate the system’s impulse response: Let an input f[x,t] be applied to the system. This input can be written as the superposition of time-varying input components f i [t]=f[x i,t] applied to each point x i :

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Response to a General Input f i [t]F i [x,t] f[x,t] Let F i [x,t] be the system’s response to this input applied to x i : For a time-invariant system we can use the impulse response to find F i [x,t] : Then, since

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Response to a General Input

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The Computational Advantage Choose a spatial mesh and a time period Calculate the impulse response over all the period to impulse inputs at possible input nodes (might be all of them) The input values at discrete points in space and time can be selected randomly, depending on the characteristics of the interconnect network (coupling, etc.) and of the interference. Let Then we can calculate the response to any such random input distribution α ij by only summation and time shifting We can explore different random input distributions easily, more flexible than experimentation

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A Demonstration Goal: Simulate response at Point F3 to a discrete-time input given by the sum of two impulses at Point 2 and two at Point 5:

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A Demonstration Theoretically, the response at point F3 to this input is obtainable by the time-shifted sum of scaled impulse responses: Calculate this analytically from the simulated impulse responses and compare with simulation result

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A Demonstration

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Sample 2-D and 3-D Networks Only 5x5 mesh shown for simplicity. All nodes connected resistively to neighbors and with an R//C to ground. Only 5x5x2 mesh shown for simplicity. Not all vertical connections shown. All nodes on the same level connected resistively to neighbors. All nodes on lowest level are connected with an R//C to ground. All nodes in intermediary levels are connected with an R//C to neighbors above and below.

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Impulse Response Solver Solves for the response over the mesh using a system of differential equations derived from KCL equations. Given here: Results for a 11x11x5 mesh. Input points: (1,1,1) (bottom southwest corner), (6,6,3) (exact center), (11,11,5) (top northeast corner). Sample output points (4,4,2) (2nd layer, southwest of center); (8,8,4) (4th layer, northeast of center).

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Impulse Response Solver Impulse response over all five layers: Impulse at (1,1,1) Impulse at (6,6,3)

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Impulse Response Solver Impulse response vs. time at output points (4,4,2) and (8,8,4).

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Impulse Response Solver Input waveform (composite of three inputs) and output waveform (composite of timeshifted/summed impulse responses).

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Conclusion Developing a method to model and investigate the response of a complex on-chip interconnect network to external RF interference or to internal coupling between different chip regions or subcircuits. Computationally advantageous: Impulse responses calculated once used to obtain system response to many inputs; Impulse responses at only the desired points in the system need to be stored to calculate the output at those points for any input waveform.

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Interconnect Network Modeling The interconnect network is a linear time invariant system: It is straightforward to calculate the output to any input distribution in space and time from the impulse responses. Example Goal: Evaluate the sensitivity of different interconnect layouts to external pulses – Obtain unit cell equivalent circuits from full wave simulation/measurement – Set up a coupled network from unit cells, emulating a certain type of interconnect network layout – Calculate the impulse responses {h i_j | i, j within chip} over time at each selected output point x i in the network, for input impulses at every possible coupling point x j – Generate a random distribution of external input pulses to find the response and create a coupling map for this type of layout – Compare different layouts (i.e. different unit cell network configurations).

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