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PCI Bus Host Bridge Registers
élanSC520 Microcontroller Register Set Manual
6-21
Programming Notes
This register is reset by a system reset or by a PCI bus reset. A PCI bus reset is initiated by setting the PCI_RST
bit in the HBCTL register (see page 6-3).
This register (PCISTACMD) is register number 1 in the host bridge-specific PCI configuration space.
The PCI Local Bus Specification, Revision 2.2, defines four bit fields in the Status/Command register that are reserved
in the élanSC520 microcontroller. These PCI bus functions do not apply to the host bridge controller:
I
Wait cycle control (bit 7): the host bridge controller does not support address/data stepping.
I
VGA palette snoop enable (bit 5): the host bridge is not a graphics device.
I
Memory write and invalidate enable (bit 4): the host bridge does not generate memory write and invalidate
commands as a PCI bus master.
I
Special cycle recognition (bit 3): the host bridge ignores PCI bus special cycles.
6
PERR_RES
Parity Error Response
This bit controls the host bridge
’s response to parity errors.
0 =The host bridge master and target controllers ignore parity errors. The host bridge treats
transactions that have a parity error (address or data) as normal transactions. In other
words, it behaves as if nothing is wrong. The D_PERR_DET bit (see page 6-20) is not set
for data parity errors, and a target abort is not issued for address parity errors.
1 =The host bridge master and target controllers report parity errors. The host bridge
responds to data parity errors by setting the D_PERR_DET bit. The host bridge target
controller responds to address parity errors by terminating the transaction with a target
abort.
The PERR_RES bit must not be changed except during PCI bus initialization after a system
reset. See the élanSC520 Microcontroller User’s Manual
, order #22004, for information
about PCI bus initialization.
5
–
3
Reserved
Reserved
This bit field should be written to 0 for normal system operation.
2
BUS_MAS
Master Enable
This enables the host bridge master controller to generate cycles on the PCI bus.
1 =The host bridge master controller is always enabled.
This bit is internally fixed to 1.
1
MEM_ENB
Memory Access Enable
This bit enables the host bridge target controller to respond to PCI bus master memory
cycles.
0 =The host bridge target controller is disabled.
1 =The host bridge target controller is enabled to respond to PCI bus master memory cycles.
The MEM_ENB bit must not be changed except during PCI bus initialization after a system
reset. See the
élanSC520 Microcontroller User’s Manual
, order #22004, for information
about PCI bus initialization.
0
IO_ENB
I/O Space Enable
This bit is normally used to enable the host bridge target controller to respond to PCI bus
master I/O cycles, however the élanSC520 microcontroller host bridge ignores all I/O cycles
from PCI bus masters.
0 =The host bridge does not respond to PCI bus I/O cycles.
This bit is internally fixed to 0.
Bit
Name
Function