1. Functional Overview
29
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
Figure 5: Reads with Conventional FIFO-Based Bridges
When Master 2 is retried in Step 2, no information is latched about the read request. When Master 2
returns for a subsequent read request in Step 4, it is treated by the bridge as the first read request.
1.6.2
PowerSpan II’s Concurrent Read Applications
1.6.2.1
PCI Host Bridge
In a PCI host bridge application, all of the PCI masters
—
for example, I/O controllers
—
potentially
receive only one retry before receiving read data. Even with another read pending, when the PCI Target
Interface of the PCI host device receives a read request, it latches the information and begins another
burst read prefetch on the processor bus. The PCI host bridge latches the addresses and delivers the
data to each master using separate, dedicated buffering. This approach greatly reduces the overall
system latency and allows for a more scalable I/O subsystem.
Master 1: Makes a
read request.
READ 1 Request
1.
Master 1: Takes the read data.
2.
READ 2 Request
Master 2: Makes a read
request and is retried.
Master 2: Makes another
read request.
4.
READ 2
READ 1
Master 1: Makes another
request and is retried.
3.
Master 2: Takes the read data.
6.
5.