
Memory-Interface Control Registers
9-13
External Bus Operation
9.3.2
Page Size Operation
Within the memory range selected by any of the four strobe lines, the ’C4x ex-
ternal interface allows you to further divide the range into pages of selected
length. This capability gives you great flexibility in the design of high-speed,
high-density memory systems combined with slower peripheral devices; each
time a page boundary is crossed, a cycle is inserted to allow external logic to
reconfigure itself.
Each PAGESIZE field in the memory interface control register (shown in
Figure 9–2 on page 9-7) works in the same manner to specify the page size
for its corresponding strobe. Table 9–3 on page 9-9 illustrates the relation-
ship between the PAGESIZE field and the bits of the address used to define
the current page and the resulting page size. Page size begins at 256 words
(with external address-bus bits 7–0 defining the address on a page, and
ranges of up to 2G words (’C40) with external address bus bits 30–0 (’C40)
defining the location on a page. The example in Figure 9–5 shows how a
pagesize field value of 10110
2
is translated into bits 30–23 defining the cur-
rent page and bits 22–0 defining an address on a page.
Figure 9–5.STRBx PAGESIZE Fields Example
30
23 22
0
External address
bus bits defining
the current page
External address
bus bits defining
address on a page
Note:
This figure represents a STRBx PAGESIZE field value of 101102 (as shown in Table 9–3).
Changing from one page to another causes a cycle to be inserted in the exter-
nal access sequence, allowing external logic to reconfigure itself appropriate-
ly. For example, the extra cycle allows time for slower devices to get off the bus,
thereby eliminating bus contention. The memory interface control logic keeps
track of the address used for the last access for each STRB. When an access
begins, the PAGE signal corresponding to the active STRB goes inactive
(high) if the access is to a new page. The PAGE0 and PAGE1 signals are inde-
pendent of one another, each having its own page-size logic.
At reset, the page-control logic is initialized so that the extra cycle is inserted
for the first access to the two strobe interfaces.
The control registers for the local memory interface function in the same way
as the control registers for the global memory interface.