FlexBus
MCF52277 Reference Manual, Rev. 1
Freescale Semiconductor
18-11
The FlexBus supports byte-, word-, longword-, and line-operand transfers and allows accesses to 8-, 16-,
and 32-bit data ports.Transfer parameters (address setup and hold, port size, the number of wait states for
the external device being accessed, automatic internal transfer termination enable or disable, and burst
enable or disable) are programmed in the chip-select control registers (CSCRs). See
“Chip-Select Control Registers (CSCR0–CSCR5)
.”
18.4.3
Data Byte Alignment and Physical Connections
The device aligns data transfers in FlexBus byte lanes with the number of lanes depending on the data port
width. The byte lane assignment is also dependent on the split bus mode setting in the CSCR
n
register.
shows the byte lanes that external memory should connect to and the sequential transfers of a
longword transfer for the supported port sizes when not in split bus mode. For example, an 8-bit memory
should connect to the single lane FB_D[31:24] (FB_BE/BWE0). A longword transfer through this 8-bit
port takes four transfers, starting with the MSB to the LSB. A longword transfer through a 32-bit port
requires one transfer on each four-byte lane of the FlexBus.
Figure 18-4. Connections for External Memory Port Sizes (CSCR
n
[SBM] = 0)
18.4.4
Bus Cycle Execution
, basic bus operations occur in four clocks:
1. S0: At the first clock edge, the address, attributes, and FB_TS are driven.
2. S1: FB_CS
n
is asserted at the second rising clock edge to indicate the device selected; by that time,
the address and attributes are valid and stable. FB_TS is negated at this edge.
For a write transfer, data is driven on the bus at this clock edge and continues to be driven until one
clock cycle after FB_CS
n
negates. For a read transfer, data is also returned at this cycle.
External slave asserts FB_TA at this clock edge.
3. S2: Read data and FB_TA are sampled on the third clock edge. FB_TA can be negated after this
edge and read data can then be tri-stated.
4. S3: FB_CS
n
is negated at the fourth rising clock edge. This last clock of the bus cycle uses what
would be an idle clock between cycles to provide hold time for address, attributes, and write data.
Data Bus
Byte 0
8-Bit Port
16-Bit Port
32-Bit Port
Byte 1
Byte 2
Byte 3
Byte 0
Byte 1
Byte 2
Byte 3
Byte 0
Byte 1
Byte 2
Byte 3
External
Memory
Memory
Memory
Byte Select
Driven with
address values
Driven with
address values
FB_BE/BWE0 FB_BE/BWE1 FB_BE/BWE2 FB_BE/BWE3
FB_D[31:24]
FB_D[23:16]
FB_D[15:8]
FB_D[7:0]
Содержание MCF52277
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