Controlling energy and process variability in System-on-Chips: needs for control theory

The design of integrated circuits, especially system-on-chips (SoC) is now constrained by many parameters such as speed, energy but also the robustness to process variability. Indeed, controlling the speed and the energy in a complex SoCs - which adopt the globally asynchronous locally synchronous (GALS) paradigm - require specific power supplies and clock generators as actuators and dedicated sensors. The problem faced by the designers is the non-uniform and non-predictable behaviour of such systems which embed several microprocessors and complex network-on-chips (NoC). In these conditions, the control laws are difficult to establish. Moreover, with the technology shrink, the control needs are increased. In order to reach an acceptable fabrication yield, the clock synchronisation - based on the assumption that the critical path is shorter than the clock period - is impracticable with large SoCs which are divided in multiple clock domains. This is why specific sensors are used to evaluate the fabrication process quality and the local environmental parameters (voltage, temperature) in each clock region in order to determine the appropriate clock frequency which does not violate the local timing constraint. All these systems are fed back and required well-suited control techniques able to manage process variability as well as energy or speed.