336
8331B–AVR–03/12
Atmel AVR XMEGA AU
27.3.1 Base Address
The base address assigned to a chip select is the lowest address in the address space, and
determines the first location in data memory space where the connected memory hardware can
be accessed. The base address associated with each chip select must be on a 4KB boundary.
Figure 27-1. Base Address
27.3.2 Address Size
The address size selects how many bits of the address should be compared when generating a
chip select. The address size can be anywhere from 256 bytes to 16MB. If the address space is
set to anything larger than 4KB, the base address must be on a boundary equal to the address
space. For example, with 1MB address space for a chip select, the base address must be on a
1MB, 2MB, etc. boundary.
If the EBI is configured so that the address spaces overlap, the internal memory space will have
priority, followed by chip select 0 (CS0), CS1, CS2, and CS3.
27.3.3 Chip Select as Address Lines
If any chip select lines are unused, these can, in some combinations, be used as address lines.
This enables larger external memory or external CS generation. Each column in Figure 27-2 on
page 336 shows enabled chip select lines (CSn) and the address lines available on unused chip
select lines (An). The right-hand column shows that all four CS lines are used as address lines
when only CS3 is enabled.
Figure 27-2. Chip Select and address line combinations
ADDRESS[23:n]
BASEADDR[23:n]
=
ADDRESS[n-1:0]
A[n-1:0]
D[7:0]
CS
CS3
CS2
CS1
CS0
CS3
CS2
CS1
A16
CS3
CS2
A17
A16
A19
A18
A17
A16
To Next Page
Loading...
Need help?
General
