External Memory
Interface
Overview
ATmega8515(L)
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)
When the eXternal MEMory (XMEM) is enabled, address space outside the internal
SRAM becomes available using the dedicated external memory pins (see Figure 1 on
page 2, Table 26 on page 66, Table 32 on page 70, and Table 38 on page 74). The
memory configuration is shown in Figure 11.
Figure 11. External Memory with Sector Select
0x0000
Internal Memory
External Memory
(0-64K x 8)
Lower Sector
SRW01
SRW00
0x25F
0x260
Upper Sector
SRL[2..0]
SRW11
SRW10
0xFFFF
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
25
2512K–AVR–01/10