[pci] Re: [oc] Re: WISHBONE serial block transfer

[Jason Silcox <jsilcox@yahoo.com>]

First time I've said anything here, so please forgive me if I
fall on my face...

I've just finished working on a design with a PCI interface...
We purchased a PCI core for use in the design and I was involved
in connecting it to the internal blocks of the design...

Burst writes were easy to handle as a Target... Just buffer the
data and don't accept a PCI read command until all data has been
released...

Burst reads were a bit tricky, but if you really understand the
PCI bus standard you'll see that a burst read and a single read
are really no different....

Basically you have 2 situations
(for this example Initiator refers to the PCI device asking for
 the data, and Target refers to the PCI core...)
1) Initiator wants one DWORD... Operation is initiated and the
   Initiator immediately indicated that this is the last byte
   it needs by deasserting the FRAME PCI bus signal.  If the
   Target doesn't abort the operation by asserting the STOP
   PCI bus signal, it puts the requested DWORD on the bus and
   asserts the TRDY PCI bus signal. The operation then completes.

2) Initiator wants more than one DWORD...  The Initiator keeps
   the IRDY and FRAME PCI bus signals asserted as long as it
   needs more data (deasserting the IRDY signal to pause the
   operation if necessary).  When the Target has a valid DWORD
   it asserts the TRDY PCI bus signal until the Initiator has
   asserted IRDY at the same time to acknowledge reception of
   the DWORD.

The point is, if you don't have data immediately, it's OK...
The Target has 16 PCI cycles to start the transfer and can
issue a Target-Retry (Master ends current operation, but can
retry later)...

What happens on the WB bus will effect the performance of
the PCI bus, but doesn't have to "indicate" a block transfer,
since block and single DWORD transfers are the same to PCI...

I guess I'm just making a point, not really answering the
question asked...

Although, I think I'll look over the WB spec and do a bit of
thinking...

Thanks for your time...
---
Jason Silcox

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