MPC563XM Reference Manual, Rev. 1
206
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
10.2.6.6
Flash Error Response Operation
The Flash array can terminate a requested access with an error. This can occur due to an uncorrectable ECC
error, an access control violation, or because of improper access sequencing during program/erase
operations. When an error response is received, the PFlash Memory Controller will not update or validate
a line read buffer. An error response can be signaled on read or write operations.
10.2.6.7
Line Read Buffers and Prefetch Operation
The PFlash Memory Controller contains four 128-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 PFlash Memory
Controller 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 16-byte boundary, and a Flash array prefetch is
initiated if the data is not already resident in a line read buffer. Prefetched data is always loaded into the
least-recently-used buffer.
Buffers can be in one of six states, listed here in prioritized order:
1. Invalid - the buffer contains no valid data
2. Used - the buffer contains valid data which has been provided to satisfy a burst type read
3. Valid - the buffer contains valid data which has been provided to satisfy a single type read
4. Prefetched - the buffer contains valid data which has been prefetched to satisfy a potential future
access
5. Busy - the buffer is currently being used to satisfy a burst read
6. 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 be loaded on a miss is based on the following replacement algorithm:
1. First, the buffers are examined to determine if there are any invalid buffers. If there are multiple
invalid buffers, the one to be used is selected using a reverse numeric priority, where buffer 0 is
selected first, then buffer 1, etc.
2. If there are no invalid buffers, the least-recently-used buffer is selected for replacement.
Once the candidate line buffer has been selected, the flash array is accessed and read data loaded into the
buffer. If the buffer load was in response to a miss, the just-loaded buffer is immediately marked as
most-recently-used. If the buffer load was in response to a speculative fetch to the next-sequential line
address after a buffer hit, the recently-used status is not changed. Rather, it is marked as most-recently-used
only after a subsequent buffer hit. This policy maximizes performance based on reference patterns of flash
accesses and allows for prefetched data to remain valid when non-prefetch enabled bus masters are granted
flash access.
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