Sun Microelectronics
86
UltraSPARC User’s Manual
Addr_Valid
is driven following the same rules as SYSADDR signals.
Addr_Valid
must be deasserted in the last cycle it is driven. The SC must
contain a holding amplifier to maintain the previously asserted state of
each Addr_Valid signal when it is undriven.
7.4.3.1 Arbitration Rules
The interface that is currently driving (or allowed to drive) SYSADDR and
Addr_Valid
is called the C
URRENT
D
RIVER
. The interface that drove (or was al-
lowed to drive) SYSADDR and Addr_Valid during the previous cycle is called
the L
AST
P
ORT
D
RIVER
. Note that the System Controller can become the C
URRENT
D
RIVER
, but it is never the L
AST
P
ORT
D
RIVER
. When SC relinquishes the control
after its transaction has completed, the value of L
AST
P
ORT
D
RIVER
is the value of
the interface that last drove the bus before the SC.
The arbitration protocol has the following rules:
1.
After reset, the UltraSPARC with port_ID<1:0>=0 is the initial L
AST
P
ORT
D
RIVER
.
2.
None of the interconnect masters or the SC may assert their requests until
44 processor cycles following the de-assertion of RESET_L.
3.
The UltraSPARC for which L
AST
P
ORT
D
RIVER
=port_ID<1:0> can take
advantage of a rule that allows request, then drive. Otherwise, the
UltraSPARC will minimally see a request, wait, then drive latency. The SC
will always see this minimal latency, since it is not included as a potential
L
AST
P
ORT
D
RIVER
.
4.
If no requests were asserted during the last cycle, the next cycle’s value for
L
AST
P
ORT
D
RIVER
remains the same as this cycle’s value.
5.
If an UltraSPARC sees that L
AST
P
ORT
D
RIVER
equals its port_id<1:0>, it
may assert its request in next cycle and drive a packet in the cycle after
that. This reduced-latency-to-drive condition is disabled if any other
requests are asserted during the cycle before request assertion.
Since the arbiter logic can use only registered requests, the reduced-
latency-to-drive condition actually would be disabled during the next
cycle, and the port would rely on the normal arbitration logic of rule 9,
which adds one more cycle of latency.
6.
The C
URRENT
D
RIVER
relinquishes ownership of the bus by deasserting its
request for one cycle in the presence of another SC or interconnect request.
This is a performance requirement.
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