
IBM21P100BGC
IBM 133 PCI-X Bridge R2.0
Configuration Registers
ppb20_pcix_regs.fm.01
October 15, 2001
5.2.5.4 Arbiter Mode Register
This register provides controls for the secondary bus arbitration logic on the bridge.
Address Offset
x‘50’
Access
See bit descriptions for details
Reset Value
x’0800’
When S_INT_ARB_EN# (pin T21) is tied low. See
for details of strapping considerations.
Reset Value
x’0801’
When S_INT_ARB_EN# (pin T21) is tied high. See
for details of strapping considerations.
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15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
Bit(s)
Access
Field Name and Description
15:8
RW
Arbiter Fairness Counter
This is the initialization value of a counter used by the internal arbiter. It controls the number of PCI bus cycles
that the arbiter holds a device’s PCI bus grant active after detecting a PCI bus request from another device. The
counter is reloaded whenever a new PCI bus grant is asserted. For every new PCI bus grant, the counter is
armed to decrement when it detects the new fall of FRAME#. If the arbiter fairness counter is set to x‘00’,the
arbiter will not remove a device’s PCI bus grant until the device has deasserted its PCI bus request.
The reset value is x‘08’.
7:2
RO
Reserved
1
RW
Broken Master Timeout Enable
This bit enables the internal arbiter to count 16 PCI bus cycles while waiting for FRAME# to become active
whenadevice’s PCI Bus Grant is active and the PCI bus is idle. If the Broken Master Timeout expires the PCI
Bus Grant for the device is de-asserted.
0
Broken Master Timeout disabled.
1
Broken Master Timeout enabled.
The reset value is b‘0’.
0
RO
External Arbiter bit
This status bit reflects whether the bridge is in internal or external arbiter mode. The value is set at reset time,
according to the polarity of the S_INT_ARB_EN# I/O pin.
0
Internal arbiter in control.
1
External arbiter in control.