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2025-06-19 17:30本頁面
【正文】 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 well as RST.REQ64 has a single pullup resistor on it and is connected to the REQ64 pin on all 64bit devices integrated onto the system board and on all 64bit PCI expansion slot. The specification states that the REQ64 signal line on each 32bit PCI expansion slot (REQ64 and ACK64 are located on the 32bit portion of the connector), however, each has its own independent pullup resistor. During reset time, the system board reset logic initially asserts the PCI RST signal while the POWERGOOD signal from the power supply is deasserted .During the assertion of RST signal. When POWERGOOD is asserted by the power supply logic, the system board reset logic deasserts the PCI RST signal. On the trailingedge of RST assertion, all 64bit devices are required to sample the stated of the REQ64 signal. All 64bit devices that are embedded on the system board or that they are installed in 64bit expansion slots sample REQ64 asserted on the trailingedge of RST. This informs them that they are connected to the extension pullups on the system board and need take no special action to keep the extension from floating when not using it. All 64－bit devices that are installed in 32bit card slot, however detect REQ64 desserted on the trailingedge of RST. This informs them that they are not connected to the system board resident pullups on the extension signal. The card logic must therefore take responsibility for the state of its own oncard 64bit extension signal lines. The card must therefore use one of the methods cited in the previous section to prevent excessive current draw by the card’s input receivers.
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