
PCI Configuration
Configuration as a Target Bridge
Copyright 1997-2000, V3 Semiconductor Inc.
EPC User
’
s Manual Revision 1.05
91
The PCI Specification describes in detail the information available during PCI configuration
and how to retrieve that information. The usage of that information is highly application
dependent and is beyond the scope of this manual.
9.1.5
Determining the Presence of Target Devices During
Configuration
A Master Abort will occur if the system host attempts to access the configuration space of a
device that is not present. This occurs, for example, when the IDSEL line for an empty PCI
slot is activated and a configuration cycle is run. When this occurs, the EPC sets the Master
Abort bit in the PCI_STATUS register and returns FFFF.FFFFH to the host. This combination
of events is used to determine the presence of PCI devices in a system following a reset.
Typically, the following mechanism is used during the boot code:
Each IDSEL line is activated in turn and a configuration read is attempted from the
Vendor ID register
If a vendor ID of FFFFH is detected it means that the current IDSEL line is not
connected to a valid device (FFFFH is a reserved Vendor ID for just this purpose)
If a valid vendor ID (i.e. not FFFFH) is detected, further configuration of the device is
performed
Most systems will elect to disable PCI error interrupts during configuration.
9.2
CONFIGURATION AS A TARGET BRIDGE
The EPC responds to type 0 configuration cycles when acting as a target bridge. Type 1
(PCI-to-PCI bridge) configuration cycles are ignored.
The following information is usually retrieved from a target device during configuration:
The number and size of I/O and memory address regions required (read from the
PCI base registers)
The vendor ID, device ID, and device type information from the PCI header
Any supplemental information provided for in the PCI header
9.2.1
EPC Base Register Response to Configuration Inquiries
Most information is read by the system host directly from the EPC
’
s configuration space
registers. For example, Vendor ID is retrieved simply by reading the Vendor ID register.