MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
12-40
Freescale Semiconductor
12.4.2.6
Small Accesses (Small Port Size and Short Burst Length)
In this context, a small access refers to an access whose burst length and port size are such that the number
of bytes requested by the internal master cannot all be fetched (or written) in one external transaction. This
is the case when the base register’s burst length bit (EBI_BR
n
[BL]) and port size bit (EBI_BR
n
[PS]) are
set such that one of two situations occur:
•
Burst accesses are inhibited and the number of bytes requested by the master is greater than the
port size (16 or 32 bit) can accommodate in a single access.
•
Burst accesses are enabled and the number of bytes requested by the master is greater than the
selected burst length (4 words or 8 words).
If this is the case, the EBI initiates multiple transactions until all the requested data is transferred. It should
be noted that all the transactions initiated to complete the data transfer are considered as an atomic
transaction, so the EBI does not allow other unrelated master accesses to intervene between the transfers.
In external master mode, this means that the EBI keeps BB asserted and does not grant the bus to another
master until the atomic transaction is complete.
shows all the combinations of burst length, port size, and requested byte count that cause the
EBI to run multiple external transactions to fulfill the request.
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 greater than 64 bits, discussed in
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.
Table 12-16. Small Access Cases
Byte Count
Requested by Internal
Master
Burst Length
Port Size
# External Accesses
to Fulfill Request
Non-Burstable Chip-Select Banks (BI=1) or Non-Chip-Select Access
4
1 beat
16-bit
2/1
1
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 12.4.2.11, “Non-Chip-Select Burst in 16-bit Data Bus Mode
” for this special DBM=1 case.
8
1 beat
32-bit
2
8
1 beat
16-bit
4
32
1 beat
32-bit
8
32
1 beat
16-bit
16
Burstable Chip-Select Banks (BI=0)
32
4 words
16-bit (8 beats), 32-bit
(4 beats)
2
Summary of Contents for MPC5553
Page 5: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 2 Freescale Semiconductor...
Page 21: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 xvi Freescale Semiconductor...
Page 47: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 1 26 Freescale Semiconductor...
Page 163: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 4 20 Freescale Semiconductor...
Page 179: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 5 16 Freescale Semiconductor...
Page 561: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 13 38 Freescale Semiconductor...
Page 615: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 14 54 Freescale Semiconductor...
Page 707: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 17 68 Freescale Semiconductor...
Page 755: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 18 48 Freescale Semiconductor...
Page 873: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 19 118 Freescale Semiconductor...
Page 984: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 Freescale Semiconductor 21 41...
Page 985: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 21 42 Freescale Semiconductor...
Page 1019: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 22 34 Freescale Semiconductor...
Page 1129: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 25 90 Freescale Semiconductor...