The Memory/Logic Interface in FPGA’s with Large Embedded Memory Arrays The Memory/Logic Interface in FPGA’s with Large Embedded Memory Arrays Steven J. E. Wilton, Member, IEEE, Jonathan Rose, Member, IEEE, and Zvonko G. Vranesic, Senior Member, IEEE Laboratory of Reliable Computing Department of Electrical Engineering National Tsing Hua University Hsinchu, Taiwan

Reference  S. J. E. Wilton, “Architectures and algorithms for field-programmable gate arrays with embedded memory,” Ph.D. dissertation, Dept. Elect. Comput. Eng., Univ. Toronto, Toronto, Ont., Canada, 1997.

Outline  Introduction  Baseline architecture  Experiment methodology and result  Enhanced architecture and its improvement

Introduction  In the past, FPGA’s have been primarily used to implement small logic subcircuits  As the capacities of FPGA’s grow, they will be use to implement much larger circuits than ever before  In order to address the storage requirement of large system, FPGA with large embedded memory arrays are now developed by many vendors

Introduction  One of the challenges when embedding memory arrays into FPGA is to provide enough interconnect between memory arrays and logic resources

Baseline Architecture

Memory/Logic Interconnect Block

Benchmark Circuit Generation  Need to generate benchmark circuit for the architecture because  Typical circuits have only a few memories each  To gather hundreds of those is not feasible  The solution is to study the types of memory configuration found in systems, and develop a stochastic memory configuration generator  Make sure they are realistic by some circuit analysis

Circuit Analysis  Memory configuration  Logic memory clustering  Interconnect patterns  Point to point patterns  Shared-connection patterns  Point to point with no shuffling patterns

Memory Configurations  171 circuits with total of 268 user memories, they are from  Recent conference proceeding  Recent journal articles  Local designer  Customer study conducted by Atera

Memory Configurations

Logic Memory Clustering

Interconnect Patterns

Stochastic Circuit Generation  A stochastic circuit generator is developed using the statistics gathered during circuit analysis  The steps of generating a benchmark circuit  Choosing logical memory configuration  Division logical memories into cluster  Choosing interconnect pattern for each cluster  Choosing number of data-in data-out subcircuits for the clusters  Generate logic subcircuits and connect them to memory arrays

Implementation Tool  Each benchmark circuit generated is “implemented” in each FPGA  Logical to physical mapping  Placement  Place memory and logic blocks simultaneously  Routing  Initially nets to memory have higher priority  Between each iteration the nets are reordered  Repeat 10 times  Increase W  Determine the minimum value of W

Memory/Logic Flexibility Result

Area Result  The area of the FPGA is the sum of  Logic blocks  Memory blocks  Routing resources  Programmable switch  Programming bits  Metal routing segments

Area Result

Delay Result  A delayed model is used to measure the memory read time of all memories in the circuit  CACTI: to estimate array access time  Elmore: address in and data out

Delay Result

Issues Issues  Nets connect more than one memory block to one or more than one logic block  When combining the small memory arrays to implement a large one  When data in pins of several user memories are driven by a common data bus  Such nets often appear but unfortunately they are hard to route, especially for larger architecture  We can use higher value of Fm for larger architecture or?

Further Investigation

Enhanced Architecture  The above motivates them to study memory to memory connection more closely  An enhanced architecture  Adding extra switches between memory arrays to support these nets  Result  Extra switches take up negligible area  Improvement in both speed and routability

Enhanced Architecture

Baseline Architecture

Enhanced Architecture

Evaluation of Enhanced Architecture  Maze routing algorithm must be restricted such that it uses memory-to-memory switches only to implement memory-to-memory connection  If the maze router is not modified…

Routing Result Using Standard Maze

Modified Maze  Even though some tracks will be wasted if a circuit contains no or few memory-to-memory connections, it alleviates the problem above

Area Result

Delay Result

Conclusion  Even with this relatively unaggressive use of the memory-to-memory switches, area is improved somewhat and speed is improved significantly  The development of algorithms that use these tracks more aggressively is left as future work  The enhanced architecture reduces the channel width by 0.5~1 tracks, and improved the speed by 25%