A Charge Flow Formulation for Guiding Analog/Mixed-Signal Placement

An analog/mixed-signal designer typically performs circuit optimization, involving intensive SPICE simulations, on a schematic netlist and then sends the optimized netlist to layout. During the layout phase, it is vital to maintain symmetry requirements to avoid performance degradation due to mismatch: these constraints are usually specified using user input or by invoking an external tool. Moreover, to achieve high performance, the layout must avoid large interconnect parasitics on critical nets. Prior works that optimize parasitics during placement work with coarse metrics such as the half-perimeter wire length, but these metrics do not appropriately emphasize performance-critical nets. The novel charge flow (CF) formulation in this work addresses both symmetry detection and parasitic optimization. By leveraging schematic-level simulations, which are available 'for free' from the circuit optimization step, the approach (a) alters the objective function to emphasize the reduction of parasitics on performance-critical nets, and (b) identifies symmetric elements/element groups. The effectiveness of the CF-based approach is demonstrated on a variety of circuits within a stochastic placement engine.