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Method to Determine State of PCI M66EN Signal When Slot is Off Disclosure Number: IPCOM000015034D
Original Publication Date: 2002-May-08
Included in the Prior Art Database: 2003-Jun-20

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The PCI 2.2 and PCI-X 1.0 and HotPlug 1.1 specs do not indicate a way to determine the state of the M66EN signal when the PCI slot is off. M66EN provides a special problem since the signal is both an output and an input. If the card does not support a bus speed of 66Mhz the card will simply ground the signal. If the card supports a bus speed of 66Mhz the card may leave the signal floating or use it as an input. The PCI HotPlug spec states that since the M66EN signal might be an input the slot needs to be powered up, the state of the signal checked, the slot powered down, the signal set in the correct position and then the slot powered up again. This recommended operating procedure leads to added complexity for the hotplug controller. This invention eliminates the requirement to powerup the slot to determine the state of the M66EN signal. The invention consists of a LM393MX comparator and two resistor divider networks. This circuit relies on the property that an input circuit to a chip will act as a resistor to ground when not powered. One leg of the comparator is set to a voltage such that the voltage across the other leg will cause the comparator to indicate if the leg is connected directly to ground. The current circuit is biased such that if the input resistance of the unpowered chip is greater than 50ohms then the circuit will indicate that the card is capable of 66Mhz operation. Resistance of less than 50ohms will falsely indicate that the card is only capable of 33Mhz operation. It is not expected that unpowered cards will appear as less than 50ohm loads. 1