Clustering the Datapath
- Stall-Enable Cluster (SEC): This cluster type passes the handshaking logic to every pipeline stage in the cluster in parallel. This means that if the cluster is stalled by logic from further down in the datapath, all logic in the SEC stalls at the same time.
- Stall Free Cluster (SFC): This cluster type adds a first in, first out (FIFO) buffer to the end of the cluster that can accommodate at least the entire latency of the pipeline in the cluster. This FIFO is often called acapacity FIFObecause the FIFO can accommodate the capacity of the cluster. Thecapacityof a cluster is the minimum number of valid data pieces that a cluster can operate on simultaneously. Capacity is always less than or equal to the latency of the datapath.Because of this FIFO, the pipeline stages in the cluster do not require any handshaking logic. The stages can run freely and drain into the capacity FIFO, even if the cluster is stalled from logic further down in the datapath.
- Bubble Handling: SECs remove only leading bubbles in the pipeline under limited circumstances. A leading bubble is a bubble that arrives before the first piece of valid data arrives in the cluster. SECs do not remove any arriving afterwards.SFCs can use the capacity FIFO to remove all bubbles from the pipeline.
- Capacity: For SECs, capacity can vary where the best-case capacity is equal to the number of register stages and the worst-case capacity is 1 because all registers in an SEC share the same stall signal. A capacity of 1 means that the cluster may be able to hold only a single piece of data, regardless of the number of pipeline stages in the cluster.This worst-case scenario corresponds to a single valid data at the end of the cluster pipeline and bubbles in the rest of the pipeline.SFCs have a capacity equal to the depth of the capacity FIFO.
- Handshaking: The capacity FIFO inside SFCs allow them to take advantage of hyper-optimized handshaking between clusters. For more information, refer to Hyper Optimized HandshakingSECs do not support this capability.