Pipelined Virtual-Channel-Based Routers - Microarchitecture of Network-on-Chip Routers - page 158

although each one incurs a negligible delay overhead, their cumulative contribution
has a considerable effect. For example, the SA unit does not simply consists of two
arbiters in series, but includes also masking and multiplexing at the input side, which
cannot be avoided or pipelined separately.

9.5.1 Three-Stage Pipelined Organization: RC|VA|SA-ST

The first 3-stage pipelined organization allows RC and VA to execute in their own
private pipeline stage and effectively get completely isolated in terms of timing from
SA and ST that execute in the same cycle.
The pipeline at the end of RC involves two possible options according to the
discussion of Sect. 5.2 . For this example, we chose to describe the organization that
adds a pipeline register both at the control path as well as the datapath, using an
intermediate pipelined EB, and offer an idle-cycle-free flow of flits. On the contrary,
the pipeline registers for the VA stage are added only in the control path following
the organization of Sect. 9.3 . The router's organization that actually implements the
chosen configuration is depicted in Fig. 9.13 .
It can be easily noticed that the derived 3-stage pipelined VC-based router
follows exactly the same organization with the single-cycle VC based router, and
the only differences appear at the registers of the state variables that now play the

Fig. 9.13 The 3-stage pipelined organization of the VC-based router that executes RC in the first
pipeline stage, VA in the second and SA, ST in the last pipeline stage. SA and ST occur in the
same cycle and they are expected to represent the critical path in terms of delay of this pipelined
configuration