6-6
MPC561/MPC563 Reference Manual
MOTOROLA
System Configuration and Protection Features
The external master may access the internal MPC561/MPC563 special registers that are
located outside the RCPU. To access one of these special purpose registers (see
EMCR[SUPU] must be cleared. The external master can then access the special register
when it is provided the address according to the MPC561/MPC563 address map. Only the
first external master access that follows EMCR setting will be assigned to the special
register map; any subsequent accesses will be directed to the normal address map. This is
done in order to enable access to the EMCR again after the required MPC561/MPC563
special register access.
Peripheral mode does not require external bus arbitration between the external master and
the internal RCPU, since the internal RCPU is disabled. The BR and BB signals should be
connected to ground, and the internal bus arbitration should be selected in order to prevent
the “slave” MPC561/MPC563 from occupying the external bus. Internal bus arbitration is
6.1.2.2
Address Decoding for External Accesses
During an external master access, the USIU compares the external address with the internal
address block to determine if MPC561/MPC563 operation is required. Since only 24 of the
32 internal address bits are available on the external bus, the USIU assigns zeros to the most
significant address bits (ADDR[0:7]).
The address compare sequence can be summarized as follows:
Normal external access. If EMCR[CONT] is cleared, the address is compared to the
— MPC561/MPC563 special register external access. If EMCR[CONT] is set by
the previous external master access, the address is compared to the
— Memory controller external access. If the first two comparisons do not match, the
internal memory controller determines whether the address matches an address
assigned to one of the regions. If it finds a match, the memory controller
generates the appropriate chip select and attribute accordingly
When trying to fetch an MPC561/MPC563 special register from an external master, the
address might be aliased to one of the external devices on the external bus. If this device is
selected by the MPC561/MPC563 internal memory controller, this aliasing does not occur
since the chip select is disabled. If the device has its own address decoding or is being
selected by external logic, this case is resolved.