
Sun Microelectronics
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7. UltraSPARC External Interfaces
7.4.2 Distributed Arbitration
The SYSADDR bus uses a distributed arbitration protocol to provide the lowest
possible latency for bus ownership, at the same time meeting the minimum cycle
time requirements of the interconnect.
The arbitration protocol has the following features:
•
Fully synchronous arbitration.
•
Distributed protocol. All contenders simultaneously calculate the next allowed
driver.
•
Round Robin among the UltraSPARC ports. Note, however, that requests from
the System Controller preempt the round robin and always get the highest
priority. The round robin among the UltraSPARC ports resumes when the SC
is finished.
•
The arbitration protocol enforces a dead cycle on the SYSADDR bus when
switching drivers. This allows sufficient time for the first driver to shut off in
the dead cycle before the next driver turns on.
•
All request signals are registered before use inside the SC or UltraSPARC. All
tristate output enables for the SYSADDR bus and Addr_Valid are registered.
This requires the protocol to be described as a pipeline, where only the state of
the request signals in the last cycle can affect the driver for the next cycle.
7.4.3 Arbitration Signals
The arbitration protocol uses the following signals for each UltraSPARC (See
Figure 7-10 on page 84):
•
Nodex_RQ
signal for the UltraSPARC’s own request
•
SC_RQ
signal for request from the system controller
•
Node_RQ<2:0>
signal for request from up to three other UltraSPARCs on
SYSADDR
•
Each UltraSPARC uses the two low order bits <1:0> from its port_ID<4:0>
pins for self identification in the arbitration algorithm. Thus, all UltraSPARCs
sharing SYSADDR must have unique values for port_ID<1:0>.
•
Addr_Valid<0..3>
. Allows the SC to indicate to a particular slave that it is the
recipient of a packet. Each UltraSPARC has a unique copy of Addr_Valid. It is
driven either by the UltraSPARC or the SC. Addr_Valid is asserted during the
first cycle of any packet.
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