Intel Itanium 2 Processor - Manual (Page 95)

Datasheet 95
Signals Reference
During power-on configuration, the priority agent must assert the BR[0]# bus signal. All
symmetric agents sample their BR[3:0]# pins on asserted-to-deasserted transition of RESET#. The
pin on which the agent samples an asserted level determines its agent ID. All agents then configure
their pins to match the appropriate bus signal protocol as shown in
Table A-6 .
A.1.15 BREQ[3:0]# (I/O)
The BREQ[3:0]# signals are the symmetric agent arbitration bus signals (called bus request). A
symmetric agent n arbitrates for the bus by asserting its BREQn# signal. Agent n drives BREQn#
as an output and receives the remaining BREQ[3:0]# signals as inputs.
The symmetric agents support distributed arbitration based on a round-robin mechanism. The
rotating ID is an internal state used by all symmetric agents to track the agent with the lowest
priority at the next arbitration event. At power-on, the rotating ID is initialized to three, allowing
agent 0 to be the highest priority symmetric agent. After a new arbitration event, the rotating ID of
all symmetric agents is updated to the agent ID of the symmetric owner. This update gives the new
symmetric owner lowest priority in the next arbitration event.
A new arbitration event occurs either when a symmetric agent asserts its BREQn# on an Idle bus
(all BREQ[3:0]# previously deasserted), or the current symmetric owner deasserts BREQn# to
release the bus ownership to a new bus owner n. On a new arbitration event, all symmetric agents
simultaneously determine the new symmetric owner using BREQ[3:0]# and the rotating ID. The
symmetric owner can park on the bus (hold the bus) provided that no other symmetric agent is
requesting its use. The symmetric owner parks by keeping its BREQn# signal asserted. On
Table A-4. BR0# (I/O), BR1#, BR2#, BR3# Signals for 4P Rotating Interconnect
Bus Signal Agent 0 Pins Agent 1 Pins Agent 2 Pins Agent 3 Pins
BREQ[0]# BR[0]# BR[3]# BR[2]# BR[1]#
BREQ[1]# BR[1]# BR[0]# BR[3]# BR[2]#
BREQ[2]# BR[2]# BR[1]# BR[0]# BR[3]#
BREQ[3]# BR[3]# BR[2]# BR[1]# BR[0]#
Table A-5. BR0# (I/O), BR1#, BR2#, BR3# Signals for 2P Rotating Interconnect
Bus Signal Agent 0 Pins Agent 3 Pins
BREQ[0]# BR[0]# BR[1]#
BREQ[1]# BR[1]# BR[0]#
BREQ[2]# Not Used Not Used
BREQ[3]# Not Used Not Used
Table A-6. BR[3:0]# Signals and Agent IDs
Pin Sampled
Asserted on
RESET#
Arbitration ID Agent ID Reported
BR[0]# 0 0
BR[3]# 1 2
BR[2]# 2 4
BR[1]# 3 6