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FlexBus
20-4
Freescale Semiconductor
Because this device shares the FlexBus signals with the PCI controller, this signal tristates between bus
cycles.
20.2.5
Read/Write (FB_R/W)
The processor drives the FB_R/W signal to indicate the current bus operation direction. It is driven high
during read bus cycles and low during write bus cycles.
Because this device shares the FlexBus signals with the PCI controller, this signal tristates between bus
cycles.
20.2.6
Address Latch Enable (FB_ALE)
The assertion of FB_ALE indicates that the device has begun a bus transaction and the address and
attributes are valid. FB_ALE is asserted for one bus clock cycle. FB_ALE may be used externally to
capture the bus transfer address (
Because this device shares the FlexBus signals with the PCI controller, this signal tristates between bus
cycles.
20.2.7
Transfer Size (FB_TSIZ[1:0])
For memory accesses, these signals, along with FB_TBST, indicate the data transfer size of the current bus
operation. The interface supports byte, word, and longword operand transfers and allows accesses to 8-,
16-, and 32-bit data ports.
For misaligned transfers, FB_TSIZ[1:0] indicates the size of each transfer. For example, if a longword
access through a 32-bit port device occurs at a misaligned offset of 0x1, a byte is transferred first
(FB_TSIZ[1:0] = 01), a word is transferred next at offset 0x2 (FB_TSIZ[1:0] = 10), and the final byte is
transferred at offset 0x4 (FB_TSIZ[1:0] = 01).
For aligned transfers larger than the port size, FB_TSIZ[1:0] behaves as follows:
•
If bursting is used, FB_TSIZ[1:0] is driven to the transfer size.
•
If bursting is inhibited, FB_TSIZ[1:0] first shows the entire transfer size and then shows the port
size.
For burst-inhibited transfers, FB_TSIZ[1:0] changes with each FB_TS assertion to reflect the next transfer
size. For transfers to port sizes smaller than the transfer size, FB_TSIZ[1:0] indicates the size of the entire
transfer on the first access and the size of the current port transfer on subsequent transfers. For example,
Table 20-2. Data Transfer Size
FB_TSIZ[1:0]
Transfer Size
00
4 bytes (longword)
01
1 byte
10
2 bytes (word)
11
16 bytes (line)
Содержание MCF54455
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