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PCI Bus Controller
Freescale Semiconductor
22-51
22.4.5.6.1
Park With Last Master
The PCI arbiter continues issuing PCI_GNT to the previous master in the absence of any requests from
other masters. There is no latency due to arbitration if the next master to request ownership of the bus is
the master that last used the bus.
22.4.5.6.2
Park With The Internal PCI Controller
The arbiter monitors for PCI bus idle state (PCI_FRAME and PCI_IRDY deasserted) before parking the
bus. The advantage to this method (versus not monitoring, where the arbiter asserts PCI_GNT to the PCI
controller to park while the current master’s transaction is active) is the current master is not forced off the
bus due to master latency time-out. For example, if a single external master requests the bus while the PCI
bus is idle, the arbiter deasserts PCI_GNT to the PCI controller and asserts PCI_GNT to the external
master a cycle later. If the master does not have another transaction to run after the first one completes, it
deasserts its PCI_REQ when it asserts PCI_FRAME.
While the transaction is in progress, the arbiter recognizes that the bus is busy and does not deassert the
current master’s PCI_GNT to park the bus with the PCI controller until the PCI bus is in an idle state
(PCI_FRAME and PCI_IRDY deasserted). The current master can continue its transaction until it
completes or until a request is received from another master. If the PCI arbiter removes PCI_GNT from
the current master and issues PCI_GNT to the PCI controller (in absence of any requests from other
masters) during the transaction, the current master is forced to relinquish the bus if its master latency timer
exhausts before all data transfers complete. It would then have to wait two clocks and re-arbitrate for the
bus again to resume the transaction at the point where it left off.
When the PCI controller comes out of reset, the internal PCI arbiter is set to park with the last master. To
prevent the bus from floating before the bus activity occurs, the arbiter automatically parks the PCI bus
ownership with the internal PCI controller. Therefore, the PCI controller drives the PCI_AD[31:0],
PCI_CBE[3:0], and PCI_PAR signals after chip reset and before the first PCI bus request.
22.4.5.7
Master Time-Out
A master is broken if it has not initiated an access (dropped PCI_FRAME) after its PCI_GNT asserts (its
PCI_REQ is also asserted) and the bus is in the idle state for 16 clocks. A 16 clock (PCI clock) timer
institutes to prevent arbitration lock-up for this case. When the timer expires, the arbiter removes the
PCI_GNT from the device and gives the bus to the master with the next highest priority. Subsequent
requests from the timed-out master are ignored until its PCI_REQ is negated for at least one clock cycle.
A status bit is set when any master times out. If the corresponding interrupt enable bit is set, a CPU
interrupt asserts. Software can query the status bits to detect a broken master in the PCI system. (See
Section 22.3.3.2, “PCI Arbiter Status Register (PASR).”
)
Table 22-34. PCI Arbiter Parking Mode Control
PACR[PKMD]
Parking Mode
0
Park with the last master to use the bus
1
Park with the internal PCI controller
Summary of Contents for MCF54455
Page 33: ...xxviii Freescale Semiconductor ...
Page 67: ...Freescale Semiconductor 1 ...
Page 125: ...Freescale Semiconductor 1 ...
Page 145: ...Enhanced Multiply Accumulate Unit EMAC 5 21 Freescale Semiconductor ...
Page 173: ...Cache 6 28 Freescale Semiconductor ...
Page 179: ...Static RAM SRAM 7 6 Freescale Semiconductor ...
Page 207: ...Power Management 9 16 Freescale Semiconductor ...
Page 323: ...Reset Controller Module 13 8 Freescale Semiconductor ...
Page 389: ...Pin Multiplexing and Control 16 44 Freescale Semiconductor ...
Page 575: ...PCI Bus Controller 22 58 Freescale Semiconductor ...
Page 600: ...Advanced Technology Attachment ATA Freescale Semiconductor 23 25 ...
Page 601: ...Freescale Semiconductor 1 ...
Page 842: ...I2 C Interface Freescale Semiconductor 33 16 ...
Page 843: ...Freescale Semiconductor 1 ...
Page 921: ...Revision History A 6 Freescale Semiconductor ...