GX1 Datasheet, PDF (176/247 Pages) National Semiconductor (TI) – Processor Series Low Power Integrated x86 Solution

Integrated Functions (Continued)
4.7.8.3 PCI Arbitration
An agent requests the bus by asserting its REQ#. Based
on the arbitration scheme set in the PCI Arbitration Control
2 register (Index 44h), the GX1 processor’s PCI arbiter will
grant the request by asserting GNT#. Figure 4-20 illus-
trates basic arbitration.
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 GX1’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. How-
ever, 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 transac-
tion, 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 com-
pletes the transaction, then deasserts FRAME# and
1
2
3
4
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.
For additional information refer to Chapter 3.4.1, Arbitration
Signaling Protocol, of the PCI Local Bus Specification,
Revision 2.1.
4.7.8.4 PCI Halt Command
Halt is a broadcast message from the GX1 processor indi-
cating it has executed a HALT instruction. The PCI Special
Cycle command is used to broadcast the message 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.
5
6
7
8
9
CLK
REQ#-a
REQ#-b
GNT#-a
GNT#-b
FRAME#
AD
ADDR
DATA
ADDR
DATA
Agent-A
Figure 4-20. Basic Arbitration
Agent-B
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