Rabbit 6000 User’s Manual
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Each of these registers provides a multiple-of-4 KB offset that is added to the logical address to provide a
physical address as shown in Figure 5.3.
Figure 5.3 MMU Operation
5.3.3 Memory Bank Operation
On startup the Rabbit 6000 checks the status of the SYSCFG pin. To provide support for external memory,
the SYSCFG pin should be set low and Memory Bank 0 enabled to use /CS0, /OE0, and /WE0 in 8-bit
mode with four wait states and write protection enabled. It is expected that an external flash device con-
taining startup code is attached to those strobes. The other memory banks come up undefined and their
controls should be set via the appropriate MBxCR register to a valid setting before use.
If SYSCFG is high, Memory Bank 0 is enabled to use /CS3, /OE0, and /WE0 in 16-bit mode. This allows
the processor to start operation directly out of the internal 1 MB RAM.
The size of the memory banks is defined in the MECR register. The default size is 256 KB (the bank selec-
tion looks at the two most significant address bits), but this value can be adjusted down to 128 KB or up to
4 MB per bank.
Each bank can be further subdivided into two equal-sized sub banks by configuring them in MBxLCR and
MBxHCR. Each sub bank can be mapped to a separate chip-select/enable combination, allowing up to
eight separate devices to be mapped in simultaneously.
The two address bits used to select the bank can be inverted in MBxCR/MBxLCR/MBxHCR, which
enables mapping different sections of a memory device larger than the current memory bank into memory.
Figure 5.4 shows an example of this feature.
Summary of Contents for 6000
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