Arbitration Logic - Microarchitecture of Network-on-Chip Routers - page 63

4.2 Round-Robin Arbitration

Round-robin arbitration logic scans the input requests in a cyclic manner, beginning
from the position that has the highest priority, and grants the first active request.
For the next arbitration cycle, the priority vector points to the position next to the
granted input. In this way, the granted input receives the lowest priority in the next
arbitration cycle.
An example of the operation of a round-robin arbiter for 4 consecutive cycles is
shown in Fig. 4.4 (the boxes labeled with a letter correspond to the active requests).
In the first cycle, input B has the highest priority to receive a grant but does not
have an active request. The inputs with an active request are inputs A and C.
The arbitration logic scans all requests in a cyclic manner starting from position
B. The first active request visited in this cyclic search is input C that is actually
granted. In the next cycle, input C should receive the lowest priority according to
the round-robin policy. Therefore, priority moves to input D. In this case, the input
that is granted is input A since it the first input with an active request when starting
searching from input D. Arbitration in the next cycles evolves in a similar manner.
Although there are several approaches for building fast round-robin arbiters like
the ones presented in Dimitrakopoulos et al. ( 2008 ) and Gupta and McKeown
( 1999 ), in this chapter we will describe another solution that leads to equally fast
arbiters and its operation is based on a simple algorithmic approach similar to the
one presented for FPA (Dimitrakopoulos et al. 2013 ). A complete overview of
all previously presented proposals regarding the logic-level design of round-robin
arbiters can be found in Dimitrakopoulos ( 2010 ).
The round-robin arbiter utilizes an N -bit priority vector P that follows the
thermometer code. As shown in the example of Fig. 4.5 , the priority vector splits
the input requests in two segments. The high-priority (HP) segment consists of the
requests that belong to high priority positions where P i D 1, while the requests,

t=0 t=1 t=2 t=3

A A A A

Search
order

B

B

C

D

C A B D

Input Granted

position with the
highest priority

Fig. 4.4 An example of the operation of a round-robin arbiter. The input granted receives the
lower priority for the next arbitration round