PCI Bus Host Bridge
9-12
Élan™SC520 Microcontroller User’s Manual
bridge waits until the write cycle has completed on the PCI bus before returning ready to
the Am5
x
86 CPU.
Write posting should not be enabled while operating in nonconcurrent arbitration mode.
See Chapter 8, “System Arbitration”, for further details on nonconcurrent mode arbitration.
9.5.3.2
Read Cycles
The PCI host bridge does not read ahead PCI bus memory for Am5
x
86 CPU read cycles.
Each Am5
x
86 CPU read cycle generates a single data phase read cycle on the PCI bus,
with only the data requested by the Am5
x
86 CPU being read. The PCI host bridge does
not burst Am5
x
86 CPU-to-PCI-bus read cycles, because the Am5
x
86 CPU typically
performs burst reads only during cache-line fills, and PCI bus memory is noncacheable.
There are a few cases when the Am5
x
86 CPU may burst two doublewords (i.e., misaligned
transfer). In this case, the PCI host bridge breaks the transfer up into single cycles on the
PCI bus.
9.5.3.3
Delayed Transaction Support
The PCI host bridge as a PCI master supports delayed transactions. A transaction that was
retried repeats until completed on the PCI bus. The PCI host bridge does not make any
distinction between a transaction that was retried and a transaction that was disconnected.
Both types of transactions are repeated until they complete on the PCI bus.
A programmable retry time-out counter prevents a deadlock condition due to a broken
target on the PCI bus. The Master Retry Time-Out (M_RETRY_TO) field in the Master Retry
Time-Out (PCIMRETRYTO) register (PCI index 41h) controls this feature. When the time-
out counter expires (a cycle was retried unsuccessfully n times on the PCI bus), the cycle
is discarded and an interrupt can be generated. For a read cycle, the data returned is all
ones. The Host Bridge Master Interrupt Address (MSTINTADD) register (MMCR offset 6Ch)
contains the address of the transaction that was retried unsuccessfully. Note that the master
retry count configuration must not be changed except during PCI bus initialization after a
system or programmable reset.
Transaction ordering is maintained during delayed transactions. A transaction that is retried
by an external PCI bus target must complete before any subsequent Am5
x
86 CPU-to-PCI
bus transactions are generated.
9.5.3.4
Host Bridge Master Bus Cycles
This section describes in detail the cycles generated by the ÉlanSC520 microcontroller
acting as PCI host bridge master and includes both the PCI bus and the internal Am5
x
86
CPU bus. Note that these are example cases only, and not all cases are shown. The
diagrams are functionally representative in nature, and should not be used to infer detailed
timing information. Note also that the synchronization between the CPU and PCI clock
domains is not shown in detail.
9.5.3.4.1
CPU Read Cycle to the PCI Bus
Figure 9-8 shows an Am5
x
86 CPU read cycle to the PCI bus. Figure 9-8 could also
represent a memory, I/O or external PCI bus device configuration cycles. The first group
of signals includes the internal Am5
x
86 CPU signals, the second group includes additional
ÉlanSC520 microcontroller internal signals, and the third group includes the PCI bus
signals. Note that the PCI bus request and grant signals are shown for convenience, but
these are not seen externally when the Am5
x
86 CPU is the initiator of PCI bus transactions.
Содержание Elan SC520
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Страница 28: ...Introduction xxviii lan SC520 Microcontroller User s Manual...
Страница 42: ...Architectural Overview 1 14 lan SC520 Microcontroller User s Manual...
Страница 78: ...System Initialization 3 22 lan SC520 Microcontroller User s Manual...
Страница 108: ...Clock Generation and Control 5 10 lan SC520 Microcontroller User s Manual...
Страница 118: ...Reset Generation 6 10 lan SC520 Microcontroller User s Manual...
Страница 148: ...System Arbitration 8 24 lan SC520 Microcontroller User s Manual...
Страница 214: ...SDRAM Controller 10 36 lan SC520 Microcontroller User s Manual...
Страница 230: ...Write Buffer and Read Buffer 11 16 lan SC520 Microcontroller User s Manual...
Страница 288: ...GP Bus DMA Controller 14 22 lan SC520 Microcontroller User s Manual...
Страница 316: ...Programmable Interval Timer 16 8 lan SC520 Microcontroller User s Manual...
Страница 328: ...Software Timer 18 4 lan SC520 Microcontroller User s Manual...
Страница 346: ...Real Time Clock 20 12 lan SC520 Microcontroller User s Manual...
Страница 360: ...UART Serial Ports 21 14 lan SC520 Microcontroller User s Manual...
Страница 414: ...AMDebug Technology 26 8 lan SC520 Microcontroller User s Manual...