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Chapter 21 Memory Protection Unit (MPU)
MPC5606BK Microcontroller Reference Manual, Rev. 2
390
Freescale Semiconductor
21.6
Functional description
In this section, the functional operation of the MPU is detailed. In particular, subsequent sections discuss
the operation of the access evaluation macro as well as the handling of error-terminated bus cycles.
21.6.1
Access evaluation macro
As previously discussed, the basic operation of the MPU is performed in the access evaluation macro, a
hardware structure replicated in the two-dimensional connection matrix. As shown in
access evaluation macro inputs the system bus address phase signals and the contents of a region descriptor
(RGDn) and performs two major functions: region hit determination (hit_b) and detection of an access
protection violation (error).
Figure 21-10. MPU access evaluation macro
is not intended to be a schematic of the actual access evaluation macro, but rather a
generalized block diagram showing the major functions included in this logic block.
21.6.1.1
Access evaluation – Hit determination
To evaluate the region hit determination, the MPU uses two magnitude comparators in conjunction with
the contents of a region descriptor: the current access must be included between the region's start and end
addresses and simultaneously the region's valid bit must be active.
Recall there are no hardware checks to verify that region's end address is greater then region's start address,
and it is software’s responsibility to properly load appropriate values into these fields of the region
descriptor.
In addition to this, the optional process identifier is examined against the region descriptor’s PID and
PIDMASK fields. In order to generate the pid_hit indication: the current PID with its PIDMASK must be
equal to the region's PID with its PIDMASK. Also the process identifier enable is take into account in this
comparison so that the MPU forces the pid_hit term to be asserted in the case of AHB bus master doesn't
provide its process identifier.
hit_b
start end
error
>
>
RGDn
System bus
hit & error
hit_b | error
r,w,x
address phase
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