Revision 1.02 – June 27, 2006
S5935 – PCI Product
Data Book
PCI Read Retries
Add-On Bus Interface
When the S5935 requests a retry for a PCI Pass-Thru
read, it indicates that the Add-On could not complete
the read in the required time. The Pass-Thru data can-
not be read by the PCI interface until the Add-On
asserts PTRDY#, indicating the access is complete.
The Pass-Thru address and data registers can be
accessed as Add-On operation registers. The inter-
face to the Pass-Thru registers is described in. The
Pass-Thru data register is updated on the rising edge
of BPCLK. For this reason, all Pass-Thru inputs must
be synchronous to BPCLK. In the following sections
the Add-On Pass-Thru interface is described for Pass-
Thru single cycle accesses, burst accesses, target-
requested retries, and when using 8-bit and 16-bit
Add-On data buses.
If the retry occurs after the Add-On has completed the
Pass-Thru operation by writing the appropriate data
into the Pass-Thru data register and asserting
PTRDY#, the S5935 asserts DEVSEL# and TRDY# to
complete the PCI read. If the Add-On still has not com-
pleted the Pass-Thru read, the S5935 waits for the
required 16 clocks. If the Add-On completes the
access during this time, TRDY# is asserted and the
access is finished. If the Add-On cannot complete the
access within 16 clocks, another retry is requested.
Single Cycle Pass-Thru Writes
A single cycle Pass-Thru write operation occurs when
a PCI initiator writes a single value to a Pass-Thru
region. PCI single cycle transfers consists of an
address phase and one data phase. During the
address phase of the PCI transfer, the S5935 stores
the PCI address into the Pass-Thru Address Register
(APTA). If the S5935 determines that the address is
within one of its defined Pass-Thru regions, it captures
the PCI data into the Pass-Thru Data Register (APTD).
When the Add-On is busy completing a Pass-Thru
read, the S5935 requests an immediate retry for all
Pass-Thru region accesses, except the region cur-
rently completing the previous access. This allows the
PCI bus to perform other operations. The next access
to the Pass-Thru region which initiated the retry must
be to the same address which caused the retry.
Another initiator accessing the same Pass-Thru region
causes the S5935 to respond with the original initia-
tor’s data (for reads). S5935 PCI Operation Registers
may be accessed while the Add-On is still completing
a Pass-Thru access. Only other Pass-Thru region
accesses receive retry requests.
Figure 1 shows a single cycle Pass-Thru write access
(Add-On read). The Add-On must read the data stored
in the APTD register and transfer it to its destination.
Note: RD# may be asserted for multiple clocks to allow
interfacing with slow Add-On devices. Data remains
valid until PTRDY# is asserted.
Figure 80. Single Cycle Pass-Thru Write
012345
BPCLK
PTATN#
PTBURST#
PTNUM[1:0]
PTWR
1
0h
PTBE[3:0]#
SELECT#
ADR[6:2]
BE[3:0]#
2Ch
0h
RD#
DQ[31:0]
PT DATA
PTRDY#
PCI Write cycle completed
Note:
For all Add-On accesses using PTADR for address data when in 16 bit mode, ADR[1] must be held low to get the low address word.
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