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# Rotating Priority and Parallel Arbitration Circuit

IP.com Disclosure Number: IPCOM000111329D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 73K

IBM

## Related People

Dieffenderfer, J: AUTHOR [+1]

## Abstract

The problem to be solved is giving multiple users access to a single resource. This is to be done on a rotating priority basis, tht is, the last user of the resource has the lowest priority to that resource when arbitration is next made. In the past, this type of arbitration has been done by providing an N-state counter (one state for each on the N users). The counter value indicates the user with the highest priority. If that user is requesting access to the single resource, its request is granted; if not, the counter is incremented by one, and the next sequential user now has highest priority. This process works, but it is a sequential arbitration method which can waste time.

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Rotating Priority and Parallel Arbitration Circuit

The problem to be solved is giving multiple users access to a
single resource.  This is to be done on a rotating priority basis,
tht is, the last user of the resource has the lowest priority to that
resource when arbitration is next made.  In the past, this type of
arbitration has been done by providing an N-state counter (one state
for each on the N users).  The counter value indicates the user with
single resource, its request is granted; if not, the counter is
incremented by one, and the next sequential user now has highest
priority.  This process works, but it is a sequential arbitration
method which can waste time.  For example, if there are five users,
the counter has a value of "1" and users 1, 2, and 3 are not
requesting the resource but user 4 is, then user 4 must wait while
the counter is incremented to a value of "4".  In many cases this
means that user 4 could not proceed with processing until it has
of the system.  The following is a method of eliminating these
"waiting periods".

Using an N-valued register, the state of the register at
arbitration time identifies the last user and therefore the priority
of all users.  The table below illustrates this where "i" is the
value in the N-state register.

First priority         i+1     * additions are modulo N
Second                 i+2
Third                  i+3
.
.
Lowest                 i

The state of the N-valued register is used as the selection
input to N minus 1 different N:1 multiplexers.  The data input to
these multiplexers is the request signals from the N different users.
If these multiplexers are numbered from 0 to N-2, then the select and
data inputs are arranged so that the output from mux #0 is the state
of the highest priority us...