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ATSAM9708

1772D–DRMSD–01/04

External Memory Timing

External Memory

Overview

The following memories can be connected to the ATSAM9708:

•

ROM or Flash memories, 16 bits wide

•

Static RAMs, 8 bits or 16 bits wide

•

DRAMs, 16 bits wide

•

SDRAMs, 16 bits wide

DRAMs and SDRAMs cannot be connected at the same time. The type of dynamic RAM

connection is determined at power-up by sensing the level of pins RAS and CAS (see

Table 4 on page 4 and “Memory Type Configuration and Boot Configuration” on page

26).

Eight-bit wide static RAM can be connected using the additional Ram Byte Select (RBS)

address signal. RBS allows access to two bytes of SRAM within one regular memory

cycle, thereby providing 16 bits of data. Eight-bit wide SRAM can be connected only

under control of WCS1. The selection 8 bits/16 bits is done by firmware.

ROM and static RAMs use linear addressing (address lines WA0 to WA26). DRAM and

SDRAMs use time-multiplexed addressing with a ROW/COL scheme (address lines

DRA0 to DRA11). Additionally, SDRAMs use the DRA0/DRA11 lines for configuration

and the DRA10 line for auto precharge.

ROM/SRAMs and DRAM/SDRAM address line share the same pins of the ATSAM9708.

The timing is determined by the input signal DRAM. If DRAM is high at the beginning of

a memory cycle, this indicates DRAM/SDRAM access.

If only one type of memory is connected (i.e., SDRAM), then the DRAM signal can be

hardwired. Otherwise, it should be derived from an external decoding of high-order

address lines.