Predator: a predictable SDRAM memory controller

Memory requirements of intellectual property components (IP) in contemporary multi-processor systems-on-chip are increasing. Large high-speed external memories, such as DDR2 SDRAMs, are shared between a multitude of IPs to satisfy these requirements at a low cost per bit. However, SDRAMs have highly variable access times that depend on previous requests. This makes it difficult to accurately and analytically determine latencies and the useful bandwidth at design time, and hence to guarantee that hard real-time requirements are met.

The main contribution of this paper is a memory controller design that provides a guaranteed minimum bandwidth and a maximum latency bound to the IPs. This is accomplished using a novel two-step approach to predictable SDRAM sharing. First, we define memory access groups, corresponding to precomputed sequences of SDRAM commands, with known efficiency and latency. Second, a predictable arbiter is used to schedule these groups dynamically at run-time, such that an allocated bandwidth and a maximum latency bound is guaranteed to the IPs. The approach is general and covers all generations of SDRAM. We present a modular implementation of our memory controller that is efficientlyintegrated into the network interface of a network-on-chip. The area of the implementation is cheap, and scales linearly with the number of IPs. An instance with six ports runs at 200 MHz and requires 0.042mm² in 0.13μm CMOS technology.