Bounding WCET of applications using SDRAM with Priority Based Budget Scheduling in MPSoCs

SDRAM is a popular off-chip memory that provides large data storage, high data rates, and is in general significantly cheaper than SRAM. There is a growing interest in using SDRAMs in safety critical application domains like aerospace, automotive and industrial automation. Some of these applications have hard real-time requirements where missing a deadline can have devastating consequence. Before integrating any hardware or software in this type of system it needs to be proven that deadlines will always be met. In practice, this is done by analyzing application´s timing behavior and calculating its Worst Case Execution Time (WCET). SDRAMs have variable access latencies depending on the refresh operation and the previous accesses. This paper builds on hardware techniques such as bank interleaving and applying Priority Based Budget Scheduling (PBS) to share the SDRAM among multiple masters. Its main contribution is a technique to bound the WCET of an application accessing a shared SDRAM of a multicore architecture using the worst case access pattern. We implemented and tested an overall memory system on an Altera Cyclone III FPGA and applied the proposed WCET estimation technique. The results show that our technique produces safe and low WCET bounds.