AT90USB1286-16MU ATMEL [ATMEL Corporation], AT90USB1286-16MU Datasheet - Page 33

• Manufacturer Part Number: AT90USB1286-16MU
• Description: Microcontroller with 64/128K Bytes of ISP Flash and USB Controller
• Manufacturer: ATMEL [ATMEL Corporation]

5.5.6
7593A–AVR–02/06
External Memory Control Register A – XMCRA
Figure 5-9.
Note:
• 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
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.
Bit
Read/Write
Initial Value
System Clock (CLK
DA7:0 (XMBK = 0)
DA7:0 (XMBK = 1)
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).
7
SRE
R/W
0
DA7:0
A15:8
External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1
CPU
ALE
WR
RD
)
Prev. addr.
Prev. data
Prev. data
Prev. data
6
SRL2
R/W
0
T1
5
SRL1
R/W
0
Address
Address
Address
T2
4
SRL0
R/W
0
XX
3
SRW11
R/W
0
Address
T3
Data
Data
Data
2
SRW10
R/W
0
T4
1
SRW01
R/W
0
AT90USB64/128
Table 5-4
T5
0
SRW00
R/W
0
and
(1)
XMCRA
T6
Figure
5-4. By
33