Datasheet

Bus Operation
6-48 MCF5206e USER’S MANUAL MOTOROLA
NOTE
TA cannot be tied to GND if the MCF5206e is not the only bus
master in the system. Damage to the part could occur if TA is
tied to GND and external master accesses using 5206e
generated termination.
6.6 MISALIGNED OPERANDS
All MCF5206e data formats can be located in memory on any byte boundary. A byte
operand is properly aligned at any address; a word operand is misaligned at an odd
address; and a longword is misaligned at an address that is not evenly divisible by four.
However, because operands can reside at any byte boundary, they can be misaligned.
Although the MCF5206e does not enforce any alignment restrictions for data operands
(including program counter (PC) relative data addressing), some performance
degradation occurs when additional bus cycles are required for longword or word
operands that are misaligned. For maximum performance, data items should be aligned
on their natural boundaries. All instruction words and extension words must reside on
word boundaries. An address error exception occurs with any attempt to prefetch an
instruction word at an odd address.
The MCF5206e converts misaligned operand accesses that are noncacheable to a
sequence of aligned accesses. Figure 6-28 illustrates the transfer of a longword operand
from a byte address to a 32-bit port, requiring more than one bus cycle. In this example,
the SIZ[1:0] signals specify a byte transfer, and the byte offset of $1. The slave device
supplies the byte and acknowledges the data transfer. When the MCF5206e starts the
second cycle, the SIZ[1:0] signals specify a word transfer with a byte offset of $2. The next
two bytes are transferred during this cycle. The MCF5206e then initiates the third cycle,
with the SIZ[1:0] signals indicating a byte transfer. The byte offset is now $0; the port
supplies the final byte and the operation is complete. Figure 6-29 is similar to the example
illustrated in Figure 6-28 except that the operand is word-sized and the transfer requires
only two bus cycles.
Figure 6-28. Example of a Misaligned Longword Transfer
Figure 6-29. Example of a Misaligned Word Transfer
TRANSFER 1
TRANSFER 2
TRANSFER 3
-
-
OP 0
OP 3
-
-
-
OP 2
-
-
OP 1
-
31 24 23 16 15 8 70
TRANSFER 1
TRANSFER 2
-
OP 0
-
-
-
-
OP 1
-
31 24 23 16 15 8 7 0
Fr
eescale S
emiconduct
or
, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
nc...