ATMEGA640 datasheet(26/449 Pages) ATMEL

26 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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)

Overview

When the eXternal MEMory (XMEM) is enabled, address space outside the internal

SRAM becomes available using the dedicated External Memory pins (see Figure 3 on

page 4, Table 39 on page 91, Table 45 on page 95, and Table 57 on page 105). The

memory configuration is shown in Figure 13.

Figure 13. External Memory with Sector Select

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 Exter-

nal Memory Control Register A – XMCRA, and the External Memory Control Register B

– XMCRB.

When the XMEM interface is enabled, the XMEM interface will override the setting in the

data direction registers that corresponds to the ports dedicated to the XMEM interface.

Memory Configuration A

0x0000

0x21FF

External Memory

(0-60K x 8)

0xFFFF

Internal memory

SRL[2..0]

SRW11

SRW10

SRW01

SRW00

Lower sector

Upper sector

0x2200

ATMEGA640 Datasheet(PDF) 26 Page - ATMEL Corporation

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