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PCI Bridging Clock 2 The master deasserts LOCK# during the address phase. This is how the locked target knows its being accessed by the master owning the lock. Only the device asserting LOCK# can release it. 3 and 4 The transaction proceeds normally. 5 If this is the last transaction in the locked series, the master releases LOCK#. 1 2 3 4 5 CLK FRAME# Release* LOCK# Continue AD Address Data IRDY# TRDY# *Target unlocks when it detects FRAME# and LOCK# deasserted Figure 8-16: Subsequent lock transactions. If a locked target sees LOCK# asserted during the address phase, a master other than the one owning the lock is attempting to access the locked target (Figure 8-17). In this case the target executes a retry abort. 145 PCI Bus Demystified 1 2 3 4 5 CLK FRAME# LOCK# Asserted by master holding lock AD Address Data IRDY# TRDY# STOP# DEVSEL# Figure 8-17: Accessing a locked target. Summary Bridging is the mechanism that allows a PCI system to expand beyond the electrical limits of a single bus segment. Bridges also serve to interface the host processor to PCI (host-to-PCI bridge) and to interface PCI to legacy busses (PCI-to-ISA bridge). Once configured, the primary job of a PCI-to-PCI bridge is to act as an address filter, accepting transactions directed at agents downstream of it and ignoring transactions that fall outside of its address windows. Bridges are allowed to prefetch read data and post write data provided they observe rules to prevent deadlocks and avoid reading stale data. Write posting can create a problem for interrupts because the interrupt may arrive at the host processor before the associated 146 PCI Bridging data buffer is written to memory. The Message Signaled Interrupt capability solves this problem by treating interrupts as bus trans-actions rather than as separate signals. The interrupt transactions are subject to the same ordering rules as data transfers so that things happen in the right order. Under rare circumstances, a master is allowed to lock a target for exclusive access. The PCI locking mechanism locks the resource and not the bus so that transactions to targets that are not locked may proceed. 147 C H A P T E R 9 CompactPCI CompactPCI is just an industrial version of the same PCI bus found in most contemporary PCs. It is electrically compatible with PCI and uses the same protocol. For reliability and ease of repair it is based on a passive backplane rather than the PC motherboard architecture. It utilizes Eurocard mechanics, made popular by VME, and a shielded pin-and-socket connector with 2mm pin spacing. Perhaps its most interesting feature is that it supports up to eight slots per bus segment rather than the four slots typically found in conventional PCI implementations. This is due to the low capaci-tance of the connector and extensive simulations that were done in the course of developing the CompactPCI spec. CompactPCI supports both 32- and 64-bit implementations at up to 33 MHz clock frequency for the full eight slots and 66 MHz over a maximum of five slots. Why CompactPCI? Advances in desktop PCs have a way of “migrating” into the world of industrial computing. In all cases the motivation is to leverage the efficiencies of scale resulting from the high volumes inherent in the desktop world. So it is with CompactPCI. 148 CompactPCI A wide range of reasonably priced PCI silicon is available for use in CompactPCI devices. VME silicon can’t begin to match the volume of PCI and so remains generally more expensive. The same considerations apply to software. Popular operating systems and applications already support PCI, particularly with respect to Plug and Play configurability. Finally, the ability to swap boards in a running system (Hot Swap) is much further developed in CompactPCI than it is in other indus-trial busses. CompactPCI is suitable for virtually any application involving industrial computing—process control, scientific instrumentation, environmental monitoring, etc. Three particular application areas Telephony Avionics Machine Vision are particularly well suited to CompactPCI implementations. The telephony industry is attracted by the low cost since they have a large number of channels to implement. They also like the high availability that comes from Hot Swap and it turns out that the 2 mm connector is already widely used in the industry. With up to 64 bits in a 3U chassis, “compact” is the key word for avionics along with high performance. Machine vision applications require the high throughput provided by PCI in a rugged industrial package. Specifications CompactPCI is embodied in a set of specifications maintained by the PCI Industrial Computer Manufacturer’s Group (PICMG) 149 ... - tailieumienphi.vn
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