AT90USB64/128
Figure 5-9. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1(1)
T1
T2
T3
T4
T5
T6
System Clock (CLKCPU
)
ALE
A15:8 Prev. addr.
DA7:0 Prev. data
WR
Address
Data
Address
Address
XX
DA7:0 (XMBK = 0) Prev. data
DA7:0 (XMBK = 1) Prev. data
RD
Data
Data
Address
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector).
The ALE pulse in period T7 is only present if the next instruction accesses the RAM (internal
or external).
5.5.6
External Memory Control Register A – XMCRA
Bit
7
6
5
4
3
2
1
0
SRE
R/W
0
SRL2
R/W
0
SRL1
R/W
0
SRL0
R/W
0
SRW11
R/W
0
SRW10
R/W
0
SRW01
R/W
0
SRW00
R/W
0
XMCRA
Read/Write
Initial Value
• Bit 7 – SRE: External SRAM/XMEM Enable
Writing SRE to one enables the External Memory Interface.The pin functions AD7:0, A15:8,
ALE, WR, and RD are activated as the alternate pin functions. The SRE bit overrides any pin
direction settings in the respective data direction registers. Writing SRE to zero, disables the
External Memory Interface and the normal pin and data direction settings are used.
• Bit 6..4 – SRL2:0: Wait-state Sector Limit
It is possible to configure different wait-states for different External Memory addresses. The
external memory address space can be divided in two sectors that have separate wait-state bits.
The SRL2, SRL1, and SRL0 bits select the split of the sectors, see Table 5-4 and Figure 5-4. By
default, the SRL2, SRL1, and SRL0 bits are set to zero and the entire external memory address
space is treated as one sector. When the entire SRAM address space is configured as one sec-
tor, the wait-states are configured by the SRW11 and SRW10 bits.
33
7593A–AVR–02/06
ATMEL [ ATMEL ]
