[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

64-bit on-board devices on 32-bit PCI bus


As per the PCI spec 2.2, sec 3.8.1, the 32-bit PCI extension has to be pulled on
the  motherboard, to prevent them from floating. Heres a situation where the
motherboard has a 32bit PCI bus . There are two on-board 64-bit PCI devices (LAN
and SCSI) on the 32-bit bus. Again, as per the PCI spec 2.2, sec 3.8., we see
that these PCI devices monitors the REQ64# signals during PCI reset de-assertion
to determine the width of the PCI bus. In the above case, these devices will see
that the REQ64# signals is de-asserted during reset de-assertion and hence
conclude that the bus width is only 32-bit. Upon determining that the bus width
is 32-bit, these devices will start driving valid bit patterns on the upper
32-bit extension in order to prevent the bus from floating. So heres the

Should i provide on-board pull-ups on the upper 32-bit extension or just leave
them unconnected?
 I personnaly feel that the 32-bit extension of these individaul devices should
be left unconnected (hoping that the devices will take care of preventing the
bus from floating). The other thing i thought if i provided a common on-board
pull-up for the 32-bit extension of both these devices is that there could be
probable data contention, because at any time and on any of the upper 32-bit
signals, the two devices could be driving opposite levels. Am i right in
thinking in this direction?

Thanks in advance.