The control bits for the External Memory Interface are located in two registers, the External Memory Control Regis-
ter 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 alter-
nate functions in section “I/O-Ports” on page 67. 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 9-3 on page 29 (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 normal 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 9-2 illustrates how to connect an external SRAM to
the AVR using an octal latch (typically “74 × 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 8MHz @ 4V and 4MHz @ 2.7V. When operating at conditions above these frequencies, the typ-
ical 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 =
5ns. Refer to tLAXX_LD/tLLAXX_ST in “External Data Memory Timing” Tables 31-11 through Tables 31-18 on pages 367
- 370. The D-to-Q propagation delay (tPD) must be taken into consideration when calculating the access time
requirement 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 9-2. External SRAM Connected to the AVR