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UM0404
The external bus interface
8.4.1
Definition of address areas
The four register pairs BUSCON4/ADDRSEL4...BUSCON1/ADDRSEL1 allow to define four
separate address areas within the address space of the ST10F276. Within each of these
address areas external accesses can be controlled by one of the four different bus modes,
independent of each other and of the bus mode specified in register BUSCON0. Each
ADDRSELx register in a way cuts out an address window, within which the parameters in
register BUSCONx are used to control external accesses.
The range start address of such a window defines the upper address bit, which are not used
within the address window of the specified size (see
).
For a given window size, only those upper address bits of the start address are used
(marked “R”), which are not implicitly used for addresses inside the window. The lower bits
of the start address (marked “x”) are disregarded.
8.4.2
Address window arbitration
For each access the EBC compares the current address with all address select registers
(programmable ADDRSELx and hard-wired XADRSx - Note that XADRS3 is programmable
also). This comparison is done in four levels.
•
The hard-wired XADRSx registers are evaluated first. A match with one of these
registers directs the access to the respective X-Peripheral using the corresponding
XBCONx register and ignoring all other ADDRSELx registers.
•
Registers ADDRSEL2 and ADDRSEL4 are evaluated before ADDRSEL1 and
ADDRSEL3, respectively. A match with one of these registers directs the access to the
respective external area using the corresponding BUSCONx register and ignoring
registers ADDRSEL1/3 (see
•
A match with registers ADDRSEL1 or ADDRSEL3 directs the access to the respective
external area using the corresponding BUSCONx register.
•
If there is no match with any XADRSx or ADDRSELx register the access to the external
bus uses register BUSCON0.
Table 30. Definition of address areas
Bit-field RGSZ
Resulting window
size
Relevant bit (R) of start address (A23...A12)
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 x x
4 Kbytes
8 Kbytes
16 Kbytes
32 Kbytes
64 Kbytes
128 Kbytes
256 Kbytes
512 Kbytes
1 Mbyte
2 Mbytes
4 Mbytes
8 Mbytes
Reserved
A23
A12
R
R R
R R
R R R
R R R
R
R
R R
R R
R R R
R R R
x
R
R R
R R
R R R
R R x
x
R
R R
R R
R R R
R x
x
x
R
R R
R R
R R R
x
x
x
x
R
R R
R R
R R x
x
x
x
x
R
R R
R R
R x
x
x
x
x
x
R
R R
R R
x
x
x
x
x
x
x
R
R R
R x
x
x
x
x
x
x
x
R
R R
x
x
x
x
x
x
x
x
x
R
R x
x
x
x
x
x
x
x
x
x
R
x
x
x
x
x
x
x
x
x
x
x