5 memory bank control registers – Jameco Electronics Rabbit 3000 User Manual
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Rabbit 3000 Microprocessor
8.5 Memory Bank Control Registers
Table 8-3 describes the operation of the four memory bank control registers. The registers
are write-only. Each register controls one quadrant in the 1M address space.
Bits 7,6—The number of wait states used in access to this quadrant. Without wait states, read requires
2 clocks and write requires 3 clocks. The wait state adds to these numbers. Wait states should only
be used for memory data accesses (RAM or data flash), not for memory from which instructions are
executed (code memory).
Bits 5, 4—These bits allow the upper address lines to be inverted. This inversion occurs after the logic
that selects the bank register, so setting these lines has no effect on which bank register is used. The
inversion may be used to install a 1M memory chip in the space normally allocated to a 256K chip.
The larger memory can then be accessed as 4 pages of 256K each. There is no effect outside the
quadrant that the memory bank control register is controlling.
Table 8-3. Memory Bank Control Register x (MBxCR = 0x014 + x)
Memory Bank x Control Register
(MB0CR)
(Address = 0x014)
(MB1CR)
(Address = 0x015)
(MB2CR)
(Address = 0x016)
(MB3CR)
(Address = 0x017)
Bit(s)
Value
Description
7:6
00
Four wait states for accesses in this bank.
01
Two wait states for accesses in this bank.
10
One wait states for accesses in this bank.
11
Zero wait states for accesses in this bank.
5
0
Pass A[19] for accesses in this bank.
1
Invert A[19] for accesses in this bank.
4
0
Pass A[18] for accesses in this bank.
1
Invert A[18] for accesses in this bank.
3:2
00
/OE0 and /WE0 are active for accesses in this bank
01
/OE1 and /WE1 are active for accesses in this bank
10
/OE0 only is active for accesses in this bank (i.e. read-only). Transactions are
normal in every other way.
11
/OE1 only is active for accesses in this bank (i.e. read-only). Transactions are
normal in every other way.
1:0
00
/CS0 is active for accesses in this bank.
01
/CS1 is active for accesses in this bank.
1x
/CS2 is active for accesses in this bank.