PCI Bus Host Bridge
9-14
Élan™SC520 Microcontroller User’s Manual
■
Clock #10: The PCI host bridge samples TRDY asserted and latches the data from the
PCI bus.
■
Clock #13: The Am5
x
86 CPU bus synchronizes the end of the PCI bus cycle and asserts
rdy to the Am5
x
86 CPU with the requested read data.
9.5.3.4.2
CPU Read Cycle to the PCI Bus with External Target Retry
Figure 9-9 shows an Am5
x
86 CPU read cycle to the PCI bus that was retried by the external
PCI bus target. An external PCI bus target can issue a retry if it is currently busy or if the
transaction will be completed as a delayed transaction.
Figure 9-9
CPU Read Cycle to the PCI Bus with External Target Retry
Notes:
The clk signal denotes the 33-MHz clock source and represents both the CPU clock and the PCI clock. This diagram
does not represent the full synchronization of signals between these clock domains.
The following sequence annotates the Am5
x
86 CPU read cycle to the PCI bus with external
target retry shown in Figure 9-9. This example is the same as a regular read (see
Section 9.5.3.4.1) until Clock #9.
■
Clock #9: The target asserts STOP with TRDY deasserted, signaling a retry. The target
may add up to 16 waitstates before asserting STOP, which would delay the PCI
transaction and Am5
x
86 CPU cycle completion.
■
Clock #10: The PCI host bridge master controller deasserts IRDY and ends the current
transaction. The data requested by the Am5
x
86 CPU was not read because of the
delayed transaction, so rdy is not returned to the Am5
x
86 CPU. The host bridge will retry
the current transaction until data is read from the target.
■
Clock #11: The PCI host bridge asserts req to re-gain access to the PCI bus. Because
the Am5
x
86 CPU is the initiator of the cycle, the bus request signal is not seen externally.
■
Clock #12: The PCI host bridge gnt signal is sampled asserted, and the PCI bus is idle,
so FRAME is asserted to retry the PCI transaction. In this example, there is no arbitration
DGGUHVV
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
GDWDLQ
E\WHHQDEOHV
UHDGFPG E\WHHQDEOHV
GDWDLQ
UHDGFPG
clk
ads
cycle_info
rdy
blast
CPU Data
pcihit
ADx
CBEx
FRAME
IRDY
TRDY
DEVSEL
STOP
req
gnt
DGGUHVV
Содержание Elan SC520
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Страница 118: ...Reset Generation 6 10 lan SC520 Microcontroller User s Manual...
Страница 148: ...System Arbitration 8 24 lan SC520 Microcontroller User s Manual...
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Страница 288: ...GP Bus DMA Controller 14 22 lan SC520 Microcontroller User s Manual...
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Страница 328: ...Software Timer 18 4 lan SC520 Microcontroller User s Manual...
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