
C
URTISS
-W
RIGHT
C
ONTROLS
E
MBEDDED
C
OMPUTING
PMC-605 PCI-P0 B
RIDGE
M
ODULE
809524 R
EVISION
D F
EBRUARY
2009
1-5
PCI-P0 B
US
A
RBITRATION
The PMC-605 can act as the bus arbiter for the PCI-P0 bus. A CPLD provides the arbitration
functions; the arbitration functions built into the 21554 bridge chip are not used.
Arbitration
Scheme
The PMC-605 uses a parallel arbitration scheme in which each card on the PCI-P0 bus is free
to request the use of the bus at any time. The PMC-605 acting as bus arbiter grants the bus
on a “first come, first serve” basis.
PCI-P0 Bus Arbiter
Enable/Disable
The PMC-605 acts as the PCI-P0 bus arbiter when the ARBDIS signal is connected directly to
Ground. When the arbiter function is enabled, REQ0# and REQ1# signals are inputs to the
arbiter, signalling that the asserting device requests the use of the PCI-P0 bus. After
completing the arbitration process, the arbiter asserts the GNT0# or GNT1# signal to the
requesting device.
The PMC-605’s arbiter function is disabled when the ARBDIS signal is connected to Vcc.
When the arbiter function is disabled, the role of REQ0# and GNT0# become reversed. The
REQ0# pin functions as GNT0# (i.e. becomes the Grant 0 input) and the GNT0# pin
functions as REQ0# (i.e. becomes the Request 0 output). In other words, REQn# signals are
always inputs and GNTn# signals are always outputs.
Figure 1.4 illustrates the function of each signal when the PMC-605’s arbiter function is either
enabled or disabled.
F
IGURE
1.4: Bus Arbitration and Signal Direction
PMC-605
21554
Arbiter/
Requester
GN
T0
#
REQ
0
#
REQ
1
#
GN
T1
#
REQ
GNT
PMC-605
21554
GN
T0
#
REQ
0
#
REQ
1
#
GN
T1
#
REQ
GNT
PMC-605
21554
GNT
0
#
RE
Q
0
#
RE
Q
1
#
GNT
1
#
REQ
GNT
Arbiter Enabled
Arbiter Disabled
Request to Enabled Arbiter
Grant from Enabled Arbiter
Request to Enabled Arbiter
Grant from Enabled Arbiter
Arbiter Disabled
(ARBDIS connected to GND)
(ARBDIS connected to Vcc)
(ARBDIS connected to Vcc)
System Slot 0
Peripheral Slot 1
Peripheral Slot 2
Arbiter/
Requester
Arbiter/
Requester
Caution
The ARBDIS signal must not be left open circuit because it is an input to the arbiter device
which does not have a pull-up on it. Inputs should not be left floating. Also note that only
one card in the system should be configured as the bus arbiter.
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