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Rabbit 6000 User’s Manual
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65
All of the power-saving modes in Chapter 36 can be used with the 16-bit mode.
The second advanced bus mode is the Page Mode. This mode also can be enabled for any external chip
select, and can be used with either 8-bit or 16-bit memories connected to these chip selects. Page-Mode
memories provide for a faster access time if the requested data are in the same page as the previous data. In
the Rabbit 6000 (and most memory devices) a page can be selected as either 8 or 16 bytes. Thus, if an
address is identical to the previous address except in the lower four bits, the access time is assumed to be
faster. These wait-state options are also controlled in the ACSxCR.
In Page Mode the chip select and /OE remain active from one page access to the next, and only the three or
four least-significant bits of the address change to request the new data. This obviously interferes with a
number of the power-saving modes and will take precedence over them for chip select accesses, as appro-
priate. The power-saving modes will still apply to the other chip select and output-enable signals. The
logic recognizes which /OE is being used with each chip select in the Page Mode.
As mentioned previously, the ACSxCR registers each contain three fields to control the generation of wait
states in the advanced bus modes. These settings are in addition to the wait-state setting in MBxCR when
an advanced bus mode is enabled. When the 16-bit bus is enabled, one to fifteen automatic wait states for
memory read bus cycles can be enabled in addition to the zero to four wait states in MBxCR. This setting
is also used for the first access when the Page Mode is enabled; a second setting selects the number of wait
states for all subsequent reads in the Page Mode, allowing from zero to three automatic wait states for the
same-page accesses in the Page Mode. The choices available for the advanced bus wait states are sufficient
to allow interfacing to a variety of standard memories for any Rabbit 6000 speed grade.
When a 16-bit memory is connected to /CS0, the first few instructions must program the device to operate
in 16-bit mode. This code is shown below. This code should be the first thing executed by your device.
Because the processor is fetching bytes from a 16-bit memory device that is not connected to A0, only
one-byte instructions can be used, and they must occur in pairs.
Summary of Contents for 6000
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Page 234: ...Rabbit 6000 User s Manual digi com 234 Figure 22 3 Sample Slow A D Converter Circuit...
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Page 420: ...Rabbit 6000 User s Manual digi com 420 Figure 37 1 Memory Read and Write Cycles...