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CHAPTER 4 BUS INTERFACE
The area selection registers (ASR1 to ASR5) and area mask registers (AMR1 to AMR5) specify
addresses for chip select areas 1 to 5.
ASR1 to ASR5 specify the upper 16 bits (A31 to A16) and AMR1 to AMR5 mask the
corresponding address bits. Each bit AMR1 to AMR5 is set to 0 for "Care" and 1 for "Don’t
care."
A bit in the Care status is set to 0 when the ASR value is 0 or to 1 when the ASR value is 1 to
indicate the address space. A bit in the Don’t care status indicates the address space for both
cases, irrespective of the ASR setting.
The following are two examples of specifying chip select areas by combining ASR and AMR.
[Example 1]
ASR1 = 00000000 00000011 B
AMR1 = 00000000 00000000 B
If the registers are set as above, the AMR1 bit corresponding to the ASR1 bit set to 1 is 0.
Therefore, the address space of area 1 will be 64KB as follows:
00000000 00000011 00000000 00000000 B (00030000 H)
to
00000000 00000011 11111111 11111111 B (0003FFFF H)
[Example 2]
ASR2 = 00001111 11111111 B
AMR2 = 00000000 00000011 B
If the registers are set as above, the ASR2 bit corresponding to the AMR2 bit set to 0 is 0, 1
(Care).
The ASR2 bit corresponding to the AMR2 bit set to 1 is 0 or 1 (Don’t care).
Therefore, the address space of area 2 will be 256KB as follows:
00001111 11111100 00000000 00000000 B (0FFC0000 H)
to
00001111 11111111 11111111 11111111 B (0FFFFFFF H)
The bus area has a total of six chip select areas.
Each area from areas 1 to 5 can be placed at an arbitrary position in a 4GB space in the
minimum unit of 64KB using ASR1 to ASR5 and AMR1 to AMR5. If external bus access is
attempted in an area specified by these registers, the corresponding chip select signal (CS0X to
CS5X) becomes low.
Area 0 is allocated to a space outside the area specified by ASR1 to ASR5 and AMR1 to AMR5.
In the reset status, an area other than 00010000H to 0005FFFFH is allocated according to the
initial values of ASR1 to ASR5 and AMR1 to AMR5.
Precaution:
Make sure that the chip select areas do not overlap.
Figure 4.3.2 compares a map set in units of 64KB according to the default setting at reset and
the area map based on Examples 1 and 2.