Integrated Functions (Continued)
4.7.8.3 PCI Arbitration
REQ#-b on CLK 6. The PCI arbiter can then grant access
to Agent A, and does so on CLK 7. Note that all buffers
must flush before a grant is given to a new agent.
An agent requests the bus by asserting its REQ#. Based
on the arbitration scheme set in the PCI Arbitration Con-
trol 2 Register (Index 44h), the GXLV processor’s PCI
arbiter will grant the request by asserting GNT#. Figure 4-
20 illustrates basic arbitration.
For additional information refer to Chapter 3.4.1, Arbitra-
tion Signaling Protocol, of the PCI Local Bus Specifica-
tion, Revision 2.1.
REQ#-a is asserted at CLK 1. The PCI arbiter grants
access to Agent A by asserting GNT#-a on CLK 2. Agent
A must begin a transaction by asserting FRAME# within
16 clocks, or the GXLV’s PCI arbiter will remove GNT#.
Also, it is possible for Agent A to lose bus ownership
sooner if another agent with higher priority requests the
bus. However, in this example, Agent B is of higher priority
than Agent A. When Agent B requests the bus on CLK 2,
Agent A is allowed to proceed per Specification. Agent A
starts its transaction on CLK 3 by asserting FRAME# and
completes its transaction. Since Agent A requests another
transaction, REQ#-a remains asserted. When FRAME# is
asserted on CLK 3, the PCI arbiter determines Agent B
should go next, asserts GNT#-b and deasserts GNT#-a
on CLK 4. Agent B requires only a single transaction. It
completes the transaction, then deasserts FRAME# and
4.7.8.4 PCI Halt Command
Halt is a broadcast message from the GXLV processor
indicating it has executed a HALT instruction. The PCI
Special Cycle command is used to broadcast the mes-
sage to all agents on the bus segment. During the
address phase of the Halt Special cycle, C/BE[3:0]# =
0001 and AD[31:0] are driven to arbitrary values. During
the data phase, C/BE[3:0]# = 1100 indicating bytes 1 and
0 are valid and AD[15:0] = 0001h.
For additional information, refer to Chapter 3.7.2, Special
Cycle, and Appendix A, Special Cycle Messages, of the
PCI Local Bus Specification, Revision 2.1.
1
2
3
4
5
6
7
8
9
CLK
REQ#-a
REQ#-b
GNT#-a
GNT#-b
FRAME#
AD
ADDR
ADDR
DATA
DATA
Agent-A
Agent-B
Figure 4-20. Basic Arbitration
Revision 1.1
175
www.national.com
NSC [ National Semiconductor ]
