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2025-06-28 17:30本頁(yè)面
【正文】 64 deasserted) and the target resides below the 4GB boundary. In this case, the initiator only uses the lower half of the bus during the address phase(because it’s only generating a 32bit address).When it discovers the currentlyaddressed target is a 32bit target(ACK64 not asserted when DEVSEL asserted),the initiator ceases to use the upper half at the bus during the data phases.. a 64bit Card in a 32bit PCI Connector Installation of a 64bit card in a 32bit card connector is permitted. The main(32bit) portion of the connector contains all of the 32bit PCI signals, which an extension to the connector contains the 64bit extension signal(with the exception of REQ64 and ACK64 which are located on the 32bit portion of the connector). When a 64bit device is installed in a 32bit PCI expansion slot, the system board pullups on AD[63::32],C/BE[7::4] and PAR64 are not available to the addin card. This means that the addin card’s input buffers that are connected to the extension signal pins will float, oscillate, and draw excessive current. The specification states that the addin card designer must not solve the problem by supplying pullup resistors on the extension lines on the addin card. Using this approach would cause problems when the card is installed in a 64bit expansion slot. There would then be tow sets of pullup resistor on these signal line (the ones on the card plus the ones on the system board).If all designer solved the problem in this manner, a machine with multiple 64bit cards inserted in 64bit card connectors would have multiple pullups on the extension signal, resulting in pullup current overload. The specification provides a method for a 64b9it card to determine at startup time whether it’s installed in a 32bit or a 64bit connector. If the card detects that it is plugged into a 64bit connector, the pullups on the system board will keep the input receives on the card from floating when the extension is not in use. On the other hand, if a y64bit card detects that it is a 32bit card connector, the logic on the card must keep the input receives from switching. The specification states that an approach similar to one of the following should be use:l Biasing the input buffer to turning it off.l Actively driving the outputs continually (since they aren’t connected to anything).. How 64bit Card Determines Type of Slot installed In When the system is powered up, the reset signal is automatically asserted. During this period of time, the logic on the system board must assert the REQ64 signal as w
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