RE:Req64# bit

["Nagesh K Vishnumurthy" <vnagesh@in.ibm.com>]

Anand,
Apart from the note I sent you also read the follwing:
Pull-Ups on AD[63:32]


      In 32-Bit PCI systems, AD[31:0] is driven at all times. When there is
      no active transaction, the PCI arbiter grants (by continuously
      asserting Gnt*) ONE of the masters (mostly the last used), thus
      having it PARKED on the bus. According to the PCI Spec., a parked
      master should actively drive AD[31:0], CBE*[3:0] and PAR (one cycle
      later).
      A 64-Bit PCI bus can accomodate 64-Bit agents as well as some good
      old 32-bitters. The point is that if a 32-Bit master is the last
      used, or for whatever reason the arbiter made a brave decision to
      have it parked when the bus is idle, NO ONE drives the upper 32 bits
      of AD (AD[63:32]). Therefore, the PCI Spec. requires Pull-Ups to be
      put over AD[63:32] and CBE*[7:4]. Following is the relevant paragraph
      from the PCI Spec:


                  When a 64-bit data path is provided, AD[63::32],
                  C/BE[7::4]#, and PAR64 require either pull-up resistors
                  or input "keepers," because they are not used in
                  transactions with 32-bit devices, and may, therefore,
                  float to the threshold level causing oscillation or high
                  power drain through the input buffer. This pull-up or
                  keeper function must be part of the motherboard central
                  resource, not the expansion board, (refer to Section
                  4.3.3.) to ensure a consistent solution and avoid pull-up
                  current overload.


      The recommended pull-up resistor value is 4K7, you may refer to
      section 4.3.3. Pull-ups in the PCI Spec. for full description and the
      formulas to calculate R(subscript: min) and R(subscript: max)

      Nagesh