Geode GXLV Processor Series Low Power Integrated x86 Solutions

Integrated Functions (Continued)4.7.8.3 PCI ArbitrationAn agent requests the bus by asserting its REQ#. Basedon the arbitration scheme set in the PCI Arbitration Control2 Register (Index 44h), the GXLV processor’s PCIarbiter will grant the request by asserting GNT#. Figure 4-20 illustrates basic arbitration.REQ#-a is asserted at CLK 1. The PCI arbiter grantsaccess to Agent A by asserting GNT#-a on CLK 2. AgentA must begin a transaction by asserting FRAME# within16 clocks, or the GXLV’s PCI arbiter will remove GNT#.Also, it is possible for Agent A to lose bus ownershipsooner if another agent with higher priority requests thebus. However, in this example, Agent B is of higher prioritythan Agent A. When Agent B requests the bus on CLK 2,Agent A is allowed to proceed per Specification. Agent Astarts its transaction on CLK 3 by asserting FRAME# andcompletes its transaction. Since Agent A requests anothertransaction, REQ#-a remains asserted. When FRAME# isasserted on CLK 3, the PCI arbiter determines Agent Bshould go next, asserts GNT#-b and deasserts GNT#-aon CLK 4. Agent B requires only a single transaction. Itcompletes the transaction, then deasserts FRAME# andREQ#-b on CLK 6. The PCI arbiter can then grant accessto Agent A, and does so on CLK 7. Note that all buffersmust flush before a grant is given to a new agent.For additional information refer to Chapter 3.4.1, ArbitrationSignaling Protocol, of the PCI Local Bus Specification,Revision 2.1.4.7.8.4 PCI Halt CommandHalt is a broadcast message from the GXLV processorindicating it has executed a HALT instruction. The PCISpecial Cycle command is used to broadcast the messageto all agents on the bus segment. During theaddress phase of the Halt Special cycle, C/BE[3:0]# =0001 and AD[31:0] are driven to arbitrary values. Duringthe data phase, C/BE[3:0]# = 1100 indicating bytes 1 and0 are valid and AD[15:0] = 0001h.For additional information, refer to Chapter 3.7.2, SpecialCycle, and Appendix A, Special Cycle Messages, of thePCI Local Bus Specification, Revision 2.1.Geode GXLV Processor Series1 2 3 4 5 6 7 8 9CLKREQ#-aREQ#-bGNT#-aGNT#-bFRAME#ADADDRDATAADDRDATAAgent-AAgent-BFigure 4-20. Basic ArbitrationRevision 1.3 175 www.national.com