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PCI Bus Controller
Freescale Semiconductor
22-49
Figure 22-42. Alternating Priority
Device 0 and device 1 assert PCI_REQ while the bus parks with device 2. Because the PCI bus is idle, the
arbiter deasserts PCI_GNT to the parked master (device 2) and a cycle later, it grants access to device 0.
Device 0’s transaction begins when PCI_FRAME asserts on clock 4. (The earliest device 0 can initiate a
transaction on the PCI bus is the cycle following PCI_GNT assertion.) It leaves its PCI_REQ asserted to
indicate it wants to perform another transaction. When PCI_FRAME asserts (PCI bus is active), hidden
arbitration occurs and PCI_GNT to device 0 deasserts on the same cycle the arbiter asserts PCI_GNT to
device 1. (Device 1 has priority because, of the two requesting masters [device 0 and device 1] device 1 is
the least recently used.)
Device 0 completes its transaction on clock 5 and relinquishes the PCI bus. On clock 6, device 1 detects
the PCI bus is idle (PCI_FRAME and PCI_IRDY deasserted) and because its PCI_GNT remains asserted,
initiates the next transaction in the next cycle. To indicate it only requires this single transaction on the PCI
bus, device 1 deasserts PCI_REQ on the same cycle it asserts PCI_FRAME.
Because device 0 is the only other requesting device, the arbiter asserts its PCI_GNT and leaves its
PCI_GNT asserted until another request is detected.
with the bus parked with device 2 while device 0 and device 1
request use of the PCI bus (Assume device 0, device 1, and device 2 are assigned the same priority group
and no other masters request use of the bus).
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]
Driven Low
ADDR
DATA
ADDR
DATA
ADDR
DATA
Access 0
Access 1
Access 0
STATE
Idle
Grant
Active
Grant
Active
Grant
Active
Turn
(Parked)
Содержание MCF54455
Страница 33: ...xxviii Freescale Semiconductor ...
Страница 67: ...Freescale Semiconductor 1 ...
Страница 125: ...Freescale Semiconductor 1 ...
Страница 145: ...Enhanced Multiply Accumulate Unit EMAC 5 21 Freescale Semiconductor ...
Страница 173: ...Cache 6 28 Freescale Semiconductor ...
Страница 179: ...Static RAM SRAM 7 6 Freescale Semiconductor ...
Страница 207: ...Power Management 9 16 Freescale Semiconductor ...
Страница 323: ...Reset Controller Module 13 8 Freescale Semiconductor ...
Страница 389: ...Pin Multiplexing and Control 16 44 Freescale Semiconductor ...
Страница 575: ...PCI Bus Controller 22 58 Freescale Semiconductor ...
Страница 600: ...Advanced Technology Attachment ATA Freescale Semiconductor 23 25 ...
Страница 601: ...Freescale Semiconductor 1 ...
Страница 842: ...I2 C Interface Freescale Semiconductor 33 16 ...
Страница 843: ...Freescale Semiconductor 1 ...
Страница 921: ...Revision History A 6 Freescale Semiconductor ...