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6.3.2
Bus Specifications
The external space bus specifications consist of three elements: bus width, number of access
states, and number of program wait states.
The bus width and number of access states for on-chip memory and registers are fixed, and are not
affected by the bus controller.
Bus Width: A bus width of 8 or 16 bits can be selected with ABWCR. An area for which an 8-bit
bus is selected functions as an 8-bit access space, and an area for which a 16-bit bus is selected
functions as a16-bit access space.
If all areas are designated for 8-bit access, 8-bit bus mode is set; if any area is designated for 16-
bit access, 16-bit bus mode is set.
Number of Access States: Two or three access states can be selected with ASTCR. An area for
which two-state access is selected functions as a two-state access space, and an area for which
three-state access is selected functions as a three-state access space.
When two-state access space is designated, wait insertion is disabled.
Number of Program Wait States: When three-state access space is designated in ASTCR, the
number of program wait states to be inserted automatically is selected with WCRH and WCRL.
From 0 to 3 program wait states can be selected.
Table 6.3 shows the bus specifications for each basic bus interface area.
Table 6.3
Bus Specifications for Each Area (Basic Bus Interface)
ABWCR ASTCR WCRH/WCRL
ABWn
ASTn
0
0
1
1
0
1
Note: n = 0 to 7
138
Wn1
Wn0
—
—
0
0
1
1
0
1
—
—
0
0
1
1
0
1
Bus Specifications (Basic Bus Interface)
Bus Width
Access States
16
2
3
8
2
3
Program Wait States
0
0
1
2
3
0
0
1
2
3
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