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PCI Bus Controller
22-50
Freescale Semiconductor
Figure 22-43. Higher Priority Override
The arbiter again deasserts device 2’s PCI_GNT on clock 2, but device 2 initiates a transaction in the same
cycle. As long as the PCI bus is idle and PCI_GNT is asserted, a master can begin a transaction on the next
cycle. (Assertion of PCI_REQ is not required.)
Next access is again awarded to device 0 and upon detection of an idle PCI state, it performs its transaction
(clocks 5 and 6). Because it has subsequent transactions to perform, device 0 leaves its PCI_REQ asserted.
Like the previous timing diagram, PCI bus ownership switches to device 1.
While device 1 performs a transaction on the PCI bus on clock 8, device 0 is the only device requesting
subsequent use of the bus. In the next cycle, the arbiter asserts PCI_GNT to device 0 in response to the
request. During that same cycle, device 2 asserts its PCI_REQ. Because access 1 remains in progress, the
arbiter determines device 2 is higher priority than device 0 (after device 1 access), rearbitrates and
deasserts PCI_GNT to device 0, and asserts PCI_GNT to device 2 in the next cycle (clock 10).
22.4.5.6
Bus Parking
Bus parking asserts PCI_GNT to a PCI master while the PCI bus is idle and there are no incoming requests.
While the bus parks on a master, the master must drive the PCI_AD, PCI_CBE, and PCI_PAR signals to
prevent these signals from floating.
The PCI arbiter can be configured to park with the last master to use the bus or park with the PCI controller.
The parking mode bit in the PCI arbiter control register, PACR[PKMD], determines the parking operation
of the arbiter as shown in
. In general, parking with the last master that acquired the bus can
gain better system performance.
PCI_CLK
PCI_REQ[0]
PCI_REQ[1]
PCI_REQ[2]
PCI_GNT[0]
PCI_GNT[1]
PCI_IRDY
PCI_FRAME
PCI_AD
0
1
2
3
4
5
6
7
8
9
10
11
12
PCI_GNT[2]
LOW
DATA
ADDR
DATA
ADDR
DATA
Access 2
Access 0
STATE
Idle
Active
Grant
Active
Grant
Turn
(Parked)
ADDR
Access 1
ADDR
DATA
Access 2
Grant
Active
Active
Содержание MCF54455
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