2. PCI Interface
42
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
An example of PowerSpan II’s concurrent read capability is illustrated in
. The concurrent
reads in the figure are represented by Read A and Read B. In the figure, both Read A and Read B are
retried. Once Read A is completed on the processor bus, Read B is initiated on the processor bus while
the contents of Read A are returned to the PCI master. Because Read B is completed on the processor
bus while Read A data is returned to the PCI bus, read latency is significantly reduced with concurrent
reads.
Figure 6: Concurrent Read Waveform
for a general discussion of read pipelining in PowerSpan II.
Concurrent Read Phases
The delayed, concurrent reads on the PCI Target consist of the following phases:
1.
Delayed Read Request
–
The PCI Target latches the transaction parameters and issues a retry.
2.
Delayed Read
–
The PCI Target obtains the requested data. The destination bus master retries requested
data.
3.
Delayed Read Completion
–
The master repeats the transaction with the same parameters used for the initial request
and data is provided by PowerSpan II.
Read line buffers are allocated on a first come, first serve basis. When an external master makes the
initial memory request, the PowerSpan II PCI Target captures the PCI address in an available delayed
read request latch. This initiates a read on the destination bus specified by the Destination Bus (DEST)
bit in the
“PCI-1 Target Image x Control Register” on page 268
.
Prefetch Reads
All PowerSpan II PCI target memory reads are considered prefetchable to 8-byte boundaries by
default. Setting the MEM_IO bit in the
“PCI-1 Target Image x Translation Address Register” on
enables 1,2,3, or 4 byte memory reads on the PCI bus and 4 byte reads on the processor bus.
When PowerSpan II is programmed to support 4 byte transactions, only 4 byte transactions are
supported. Burst transactions are not supported while the MEM_IO bit is set to 1.
Processor Bus
PCI Bus
RET-A
RET-B
RET-A
RET-B
ACC-A
Completion-A
ACC-B
Completion-B
PPC-RDB
PPC-RDA
Device,
Arbitration and
Memory Latency
Arbitration and
Memory Latency
Single Read Latency
Double Read Latency