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Chapter 5 Memory Management
67
All of the power-saving modes in Chapter 29 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 5000 (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 prece-
dence over them for chip select accesses, as appropriate. 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 seven 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 memo-
ries for any Rabbit 5000 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.
ORG
0000h
XOR
A
; a <= 00000000
XOR
A
LD
H, A
; h <= 00000000
LD H,
A
SCF
SCF
RLA
; a <= 00000001
RLA
; a <= 00000010
LD
B, A
; b <= 00000010
LD B,
A
SCF
SCF
ADC
A, B
; a <= 00000101
ADC
A, B
; a <= 00000111
ADD
A, A
; a <= 00001110
Содержание Rabbit 5000
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