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External Bus Interface (EBI)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
317
In most cases, the timing for small accesses is the same as for normal single-beat and burst accesses, except
that multiple back-to-back external transfers are executed for each internal request. These transfers have
no additional dead cycles in-between that are not present for back-to-back stand-alone transfers except for
the case of writes with an internal request size of > 64 bits, discussed in
Section 14.5.2.6.2, Small access
example #2: 32-byte write with external TA
.
The following sections show a few examples of small accesses. The timing for the remaining cases in
can be extrapolated from these and the other timing diagrams in this document.
14.5.2.6.1
Small access example #1: 32-bit write to 16-bit port
shows an example of a 32-bit write to a 16-bit port, requiring two 16-bit external transactions.
Figure 14-27. Single Beat 32-bit Write Cycle, 16-bit Port Size, Basic Timing
14.5.2.6.2
Small access example #2: 32-byte write with external TA
shows an example of a 32-byte write to a non-chip-select device using external TA, requiring
eight 32-bit external transactions. Note that due to the use of external TA, RD_WR does not toggle
32
4 words
16-bit (8 beats), 32-bit
(4 beats)
2
1
In 32-bit data bus mode (DBM=0 in EBI_MCR), two accesses are performed. In 16-bit data bus mode
(DBM=1), one 2-beat burst access is performed and this is not considered a “small access” case.
See
Section 14.5.2.9, Non-chip-select burst in 16-bit data bus mode
for this special DBM=1 case.
2
Only supported for case of 32-bit internal AMBA data bus.
3
Only supported for case of 64-bit internal AMBA data bus.
Table 14-13. Small Access Cases (continued)
Byte Count
Requested by internal
master
Burst Length
Port Size
# External Accesses
to Fulfill Request
DATA is valid
DATA is valid
CLKOUT
ADDR[3:31]
TS
TA
RD_WR
BDIP
WE
CS[
n
]
A
A + 2
ABCDXXXX
EFGHXXXX
DATA[0:31]
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