ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 26

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

ATmega103 Compatibility
Using the External Memory
Interface
Address Latch Requirements
26
ATmega64(L)
Both External Memory Control Registers, XMCRA and XMCRB, are placed in Extended
I/O space. In ATmega103 compatibility mode, these registers are not available, and the
features selected by these registers are not available. The device is still ATmega103
compatible, as these features did not exist in ATmega103. The limitations in
ATmega103 compatibility mode are:
•
•
•
•
•
The interface consists of:
•
•
•
•
•
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 64. 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.
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:
•
•
•
The external memory interface is designed to guaranty minimum address hold time after
G is asserted low of t
page 338). The D to Q propagation delay (t
calculating the access time requirement of the external component. The data setup time
Only two wait-state settings are available (SRW1n = 0b00 and SRW1n = 0b01).
The number of bits that are assigned to address high byte are fixed.
The external memory section cannot be divided into sectors with different wait-state
settings.
Bus Keeper is not available.
RD, WR, and ALE pins are output only (Port G in ATmega64).
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.
D to Q propagation delay (t
Data setup time before G low (t
Data (address) hold time after G low (
h
= 5 ns (refer to t
pd
).
su
).
LAXX_LD
th
).
pd
) must be taken into consideration when
/t
LLAXX_ST
in Table 138 to Table 145 on
2490G–AVR–03/04