NXP Semiconductors MPC5566 - FBIU Line Read Buffers and Prefetch Operation

Flash Memory
MPC5566 Microcontroller Reference Manual, Rev. 2
13-24 Freescale Semiconductor
13.4.1.7 FBIU Line Read Buffers and Prefetch Operation
The Flash BIU contains a pair of 256-bit line read buffers which are used to hold data read from the flash
array. Each buffer operates independently and is filled using a single array access. The buffers are used for
both prefetch and normal demand fetches.
Prefetch triggering is controllable on a per-master and access-type basis. Bus masters can be enabled or
disabled from triggering prefetches, and triggering can be further restricted based on whether a read access
is for instruction or data and whether or not it is a burst access. A read access to the Flash BIU can trigger
a prefetch to the next sequential line of array data on the cycle following the request. The access address
is incremented to the next-higher 32-byte boundary, and a flash array prefetch is initiated if the data is not
already resident in a line read buffer. Prefetched data is loaded into the buffer which is not being used to
satisfy the original request.
Buffers can be in one of six states, listed here in prioritized order:
• Invalid—the buffer contains no valid data.
• Used—the buffer contains valid data which has been provided to satisfy a burst type read.
• Valid—the buffer contains valid data which has been provided to satisfy a single type read.
• Prefetched—the buffer contains valid data which has been prefetched to satisfy a potential future
access.
• Busy—the buffer is currently being used to satisfy a burst read.
• Busy fill—the buffer has been allocated to receive data from the flash array, and the array access
is still in progress.
Selection of a buffer to fill on a buffer miss is based on this prioritized order beginning with the first item
(invalid). Selection of a buffer to fill on a triggered prefetch is based on the buffer which is not being used
to satisfy the triggering access.
The consequences of this replacement policy are that buffers are selected for filling on a ‘least recently
updated’ basis when prefetching, and on a ‘most recently emptied’ basis for demand fetches (that is, a fetch
which is actually satisfying a current system bus access). This policy allows for prefetched data to remain
valid when non-prefetch enabled bus masters are granted flash access.
Several algorithms are available for prefetch control which trade off performance for power. They are
described in Section 13.3.2.8, “Flash Bus Interface Unit Control Register (FLASH_BIUCR).” More
aggressive prefetching increases power due to the number of wasted (discarded) prefetches, but can
increase performance by lowering average read latency.
13.4.1.8 FBIU Instruction/Data Prefetch Triggering
Prefetch triggering can be enabled for instruction reads. Triggering can be enabled for all instruction reads
or only for instruction burst reads. Prefetch triggering can be enabled for data reads. Triggering can be
enabled for all data reads or only for data burst reads. Prefetches are not triggered by write cycles.