The control bits for the External Memory Interface are located in three registers, the
MCU Control Register – MCUCR, the External 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.
For details about the port override, see the alternate functions in section “I/O Ports” on
page 65. The XMEM interface will auto-detect whether an access is internal or external.
If the access is external, the XMEM interface will output address, data, and the control
signals on the ports according to Figure 13 (this figure shows the wave forms without
wait-states). When ALE goes from high-to-low, there is a valid address on AD7:0. ALE is
low during a data transfer. When the XMEM interface is enabled, also an internal access
will cause activity on address, data and ALE ports, but the RD and WR strobes will not
toggle during internal access. When the External Memory Interface is disabled, the nor-
mal pin and data direction settings are used. Note that when the XMEM interface is
disabled, the address space above the internal SRAM boundary is not mapped into the
internal SRAM. Figure 12 illustrates how to connect an external SRAM to the AVR using
an octal latch (typically “74 x 573” or equivalent) which is transparent when G is high.
Address Latch Requirements
Due to the high-speed operation of the XRAM interface, the address latch must be
selected with care for system frequencies above 8 MHz @ 4V and 4 MHz @ 2.7V.
When operating at conditions above these frequencies, the typical old style 74HC series
latch becomes inadequate. The External Memory Interface is designed in compliance to
the 74AHC series latch. However, most latches can be used as long they comply with
the main timing parameters. The main parameters for the address latch are:
• D to Q propagation delay (tPD).
• Data setup time before G low (tSU).
• Data (address) hold time after G low (TH).
The External Memory Interface is designed to guaranty minimum address hold time
after G is asserted low of th = 5 ns. Refer to tLAXX_LD/tLLAXX_ST in “External Data Memory
Timing” Tables 137 through Tables 144 on pages 331 - 333. The D-to-Q propagation
delay (tPD) must be taken into consideration when calculating the access time require-
ment of the external component. The data setup time before G low (tSU) must not
exceed address valid to ALE low (tAVLLC) minus PCB wiring delay (dependent on the
capacitive load).
Figure 12. External SRAM Connected to the AVR
AD7:0
ALE
AVR
A15:8
RD
WR
DQ
G
D[7:0]
A[7:0]
SRAM
A[15:8]
RD
WR
28 ATmega128
2467O–AVR–10/06