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

["Weng Tianxiang" <WTX@umem.com>]

Hi Neal,
I disagree with you.

PCI Specs 2.2, page 101:
64-bit transactions on PCI are dynamically negotiated(once per
transaction) between the Target and Master, ...

If a motherboard is stick to your option 1, no ways for other 64-bit
boards in a system to do any 64-bit transaction.

A motherboard should use your option 2 and IT WILL IMPROVE SYSTEM
PERFORMANCE for ANY 64-BIT BOARDS.

Weng

-----Original Message-----
From: Neal Palmer [mailto:neal@dinigroup.com] 
Sent: Tuesday, July 30, 2002 11:54 PM
To: Venkateshwarlu V
Cc: Anand.Kuriakose@smartm.com; pci-sig@znyx.com; Gopal Savanur
Subject: Re: 64-bit on-board devices on 32-bit PCI bus



Anand,

  You are making a motherboard right?  You have 2 options, both of which

are valid:

1) connect ONLY the 32-bit signals between all 3 parts.  Keep the 64-bit

signals disconnected (no connections between any devices).  Do not 
assert REQ64# during RST#.  The two 64-bit devices will automatically 
drive the upper 32-bits of data and CBE to some non-floating state.
This 
doesn't require any extra resistors.

2) Connect the devices with the full bus width.  And put down some logic

to drive REQ64# active during RST#.  You will have to add pullup
resistors 
for upper AD and CBE lines.  But note that this will NOT improve system 
performance because the SCSI and network cards will ALWAYS access host 
memory which is connected to the PCI bus through a 32-bit chip (i.e. all

of your transactions to host memory will have ACK64# deasserted).

  So, you should probably just go with option #1 (unless you are going
to 
write special SCSI or network drivers to send the data to a different 
64-bit card plugged into an expansion slot).

-- Neal

On Wed, 31 Jul 2002, Venkateshwarlu V wrote:

> ** Proprietary **
> 
> Hello Mr.Anand,
> I will try to answer ur questions here.
> 
> Should i provide on-board pull-ups on the upper 32-bit extension or 
> just leave them unconnected? U should not provide any on-board 
> pull-ups, incase if u use same extension board for 64-bit connector, 
> there are pull-ups for all the data lines on mother-board also, due to

> two pull-ups on each data line, there will be more pull-up current. So

> upper 32-bits must be 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 device cannot take care of preventing 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?
> Sol'n: Configure Ur device in output mode(upper 32-bits), when u
insert it in 32-bit slots(according to REQ64# logic), drive zero/one
onto the upper 32-bit bus.
> 
> 
> 
> >>> <Anand.Kuriakose@smartm.com> 07/30/02 11:00PM >>>
> 
> 
> Hi,
> 
> 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 question.
> 
> 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.
> 
> Anand.
> 
> 
> 

-- 
-- Neal Palmer

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