AT90USB64/128
5.5
External Memory Interface
With all the features the External Memory Interface provides, it is well suited to operate as an
interface to memory devices such as External SRAM and Flash, and peripherals such as LCD-
display, A/D, and D/A. The main features are:
• Four different wait-state settings (including no wait-state).
• Independent wait-state setting for different external Memory sectors (configurable sector size).
• The number of bits dedicated to address high byte is selectable.
• Bus keepers on data lines to minimize current consumption (optional).
5.5.1
Overview
When the eXternal MEMory (XMEM) is enabled, address space outside the internal SRAM
becomes available using the dedicated External Memory pins (see Figure 2-1 on page 5, Table
10-3 on page 80, and Table 10-9 on page 84). The memory configuration is shown in Figure 5-4.
Figure 5-4. External Memory with Sector Select
M
e
m
or
y
C
onf
igu
r
a
t
ion
A
0x0000
I
nternal memory
ISRAM end
XMem start
Lower sector
S
S
R
R
W
W
01
00
SRL[2..0]
E
xt
e
-
r
na
60
l
K
Memory
x 8)
U
ppe
r
s
e
cto
r
(
0
S
S
R
R
W
W
11
10
0xFFFF
5.5.2
Using the External Memory Interface
The interface consists of:
• AD7:0: Multiplexed low-order address bus and data bus.
• A15:8: High-order address bus (configurable number of bits).
• ALE: Address latch enable.
• RD: Read strobe.
• WR: Write strobe.
The control bits for the External Memory Interface are located in two registers, the External
Memory Control Register A – XMCRA, and the External Memory Control Register B – XMCRB.
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7593A–AVR–02/06
ATMEL [ ATMEL ]
