PCI Bus Host Bridge
9-28
Élan™SC520 Microcontroller User’s Manual
9.5.6
Latency
PCI bus latency issues are described separately for the CPU and external PCI bus masters.
■
Master latency refers to the case when the ÉlanSC520 microcontroller’s Am5
x
86 CPU
is the master on the PCI bus.
■
Target latency refers to the case when the ÉlanSC520 microcontroller is a PCI bus target
accessed by external PCI bus masters.
9.5.6.1
Master Latency
The posted write buffer allows Am5
x
86 CPU memory-write cycles to complete without
incurring the PCI bus transaction latency. Any other cycle between the CPU and the PCI
bus (memory read, I/O write, I/O read) must complete on the PCI bus before ready is
returned to the Am5
x
86 CPU. Note that write posting must be disabled while the ÉlanSC520
microcontroller is operating in nonconcurrent arbitration mode. See Chapter 8, “System
Arbitration”, for details on nonconcurrent mode arbitration.
The target being accessed may retry the Am5
x
86 CPU cycle (target busy) multiple times,
which would delay the Am5
x
86 CPU. This performance penalty can be limited by
configuration of the Am5
x
86 CPU using the Master Retry Time-Out (M_RETRY_TO) field
in the Master Retry Time-Out (PCIMRETRYTO) register (PCI index 41h), which limits the
number of times the PCI host bridge retries a transaction before returning the rdy signal to
the Am5
x
86 CPU. Note that the master retry count configuration must not be changed
except during PCI bus initialization after a system or programmable reset.
The Am5
x
86 CPU typically performs non-burst read transactions to the PCI bus, because
PCI bus memory is noncacheable (write transactions to PCI are
always non-burst). There
are a few cases when the CPU bursts up to two doublewords on a read transaction. For
simplicity, in these cases, the PCI host bridge breaks up any Am5
x
86 CPU burst read cycles
into single doubleword read transactions on the PCI bus, which also slows down the Am5
x
86
CPU read performance to the PCI bus. Because the PCI host bridge master controller
performs single data phase transactions only, the master latency timer is not implemented.
9.5.6.2
Target Latency
Write posting and delayed transactions in the PCI host bridge target controller allow external
PCI bus master cycles to complete without incurring SDRAM access latency. Without write
posting and delayed transactions, the PCI host bridge target controller would insert wait
states, while arbitrating for use of the SDRAM controller.
Delayed transaction support allows this time spent arbitrating for the CPU bus and the
SDRAM controller transaction to be reallocated to another bus master, rather than forcing
the first bus master to remain in a long wait state period. Instead, the first bus master’s
request is latched and placed in the delayed transaction queue for processing by the PCI
host bridge, and the bus master is forced off of the PCI bus with a retry, at which point the
PCI bus arbiter may grant the bus to another PCI bus master. The second PCI bus master
could perform a peer-to-peer transfer or memory write to SDRAM while the PCI host bridge
continues to process the first bus master’s request.
Delayed transactions avoid the wasted bus bandwidth that may occur if the PCI host bridge’s
response to the transaction exceeded the specified 32 PCI bus clocks (16 for non-host
bridge devices), at which point the PCI bus master would be retried anyway (thus wasting
16–32 PCI bus clocks).
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...