AT90USB646 datasheet(30/435 Pages) ATMEL

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7593D–AVR–07/06

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 81, and Table 10-9 on page 85). The memory configuration is shown in Figure 5-4.

Figure 5-4.

External Memory with Sector Select

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.

emory Configuration A

x0000

xternal Memory

(0-60K

x 8)

xFFFF

nternal memory

SRL[2..0]

SRW11

SRW10

SRW01

SRW00

ower sector

pper sector

ISRAM end

XMem start

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