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FlexBus
Freescale Semiconductor
20-15
20.4.6.1
Basic Read Bus Cycle
During a read cycle, the ColdFire device receives data from memory or a peripheral device.
is a read cycle flowchart.
NOTE
Throughout this chapter FB_AD[31:
X
] indicates a 32-, 16-, or 8-bit wide
data bus. FB_AD[
] is an address bus that can be
-bits in
width.
Figure 20-6. Read Cycle Flowchart
The read cycle timing diagram is shown in
.
NOTE
In the next set of timing diagrams, the dotted lines indicate FB_TA, FB_OE,
and FB_CS
n
timing when internal termination is used (CSCR[AA] = 1).
The external and internal FB_TA assert at the same time; however, FB_TA
is not driven externally for internally-terminated bus cycles.
NOTE
The processor drives the data lines during the first clock cycle of the transfer
with the full 32-bit address. This may be ignored by standard connected
devices using non-multiplexed address and data buses. However, some
applications may find this beneficial.
The address and data busses are muxed between the FlexBus and PCI
controller. At the end of the read bus cycles the address signals are
indeterminate.
Y
:0
32-, 24-, or 16
1. Select the appropriate slave device.
Assert FB_TA (external termination).
3.
1. Negate FB_TA (external termination).
1. Decode address.
1. Set FB_R/W to read.
Assert FB_CS
n
.
2.
1.
(auto-acknowledge/internal termination).
Sample FB_TA low and latch data.
2.
1. Start next cycle.
ColdFire device
System
2. Place address on FB_AD[31:0].
2. Drive data on FB_AD[31:
X
].
3. Assert FB_ALE.
1. Negate FB_ALE.
FlexBus asserts internal FB_TA
Содержание MCF54455
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