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Memory Protection Unit (MPU)
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 18-15
18.4 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 AHB bus cycles.
18.4.1 Access Evaluation Macro
As 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
Figure 18-11 , the access
evaluation macro inputs the AHB system bus address phase signals (AHB_ap) and the contents of a region
descriptor (RGD
n ) and performs two major functions: region hit determination (
hit_b
) and detection of
an access protection violation (
error
).
Figure 18-11. MPU Access Evaluation Macro
Figure 18-11 is not a schematic of the actual access evaluation macro, but a generalized block diagram
showing the major functions included in this logic block.
M0SM Bus Master 0 Supervisor Mode Access Control. This 2-bit field defines the access controls for bus master ID 0
(e200z6) when operating in supervisor mode. The M0SM field is defined as:
00
r, w, x =
read, write and execute allowed.
01
r, –, x =
read and execute allowed, but no write.
10
r, w, – =
read and write allowed, but no execute.
11 Same access controls as that defined by M0UM for user mode.
M0UM Bus Master 0 User Mode Access Control. This 3-bit field defines the access controls for bus master ID 0 (e200z6)
when operating in user mode. The M0UM field consists of three independent bits, enabling read, write, and execute
permissions:
{r,w,x}
. If set, the bit allows the given access type to occur; if cleared, an attempted access of that
mode may be terminated with an access error (if not allowed by any other descriptor) and the access not performed.
Table 18-10. MPU_RGDAACn Field Descriptions (continued)
Field Description
hit_b
start end
error
> >
RGDn
AHB_ap
hit & error hit_b | error
>=
<=
r,w,x