AT90USB646 Datasheet, PDF (31/434 Pages) ATMEL Corporation – Microcontroller with 64/128K Bytes of ISP Flash and USB Controller

AT90USB64/128
5.5.4
5.5.5
Pull-up and Bus-keeper
The pull-ups on the AD7:0 ports may be activated if the corresponding Port register is written to
one. To reduce power consumption in sleep mode, it is recommended to disable the pull-ups by
writing the Port register to zero before entering sleep.
The XMEM interface also provides a bus-keeper on the AD7:0 lines. The bus-keeper can be dis-
abled and enabled in software as described in “External Memory Control Register B – XMCRB”
on page 34. When enabled, the bus-keeper will keep the previous value on the AD7:0 bus while
these lines are tri-stated by the XMEM interface.
Timing
External Memory devices have different timing requirements. To meet these requirements, the
XMEM interface provides four different wait-states as shown in Table 5-5. It is important to con-
sider the timing specification of the External Memory device before selecting the wait-state. The
most important parameters are the access time for the external memory compared to the set-up
requirement. The access time for the External Memory is defined to be the time from receiving
the chip select/address until the data of this address actually is driven on the bus. The access
time cannot exceed the time from the ALE pulse must be asserted low until data is stable during
a read sequence (See t_LLRL+ t_RLRH - t_DVRH in Tables 30-6 through Tables 30-13 on pages 408 -
411). The different wait-states are set up in software. As an additional feature, it is possible to
divide the external memory space in two sectors with individual wait-state settings. This makes it
possible to connect two different memory devices with different timing requirements to the same
XMEM interface. For XMEM interface timing details, please refer to Tables 30-6 through Tables
30-13 and Figure 30-7 to Figure 30-10 in the “External Data Memory Timing” on page 408.
Note that the XMEM interface is asynchronous and that the waveforms in the following figures
are related to the internal system clock. The skew between the internal and external clock
(XTAL1) is not guarantied (varies between devices temperature, and supply voltage). Conse-
quently, the XMEM interface is not suited for synchronous operation.
Figure 5-6. External Data Memory Cycles without Wait-state (SRWn1=0 and SRWn0=0)
T1
T2
T3
T4
System Clock (CLK_CPU)
ALE
A15:8 Prev. addr.
Address
DA7:0 Prev. data
Address XX
Data
WR
DA7:0 (XMBK = 0) Prev. data
Address
Data
DA7:0 (XMBK = 1) Prev. data
Address
Data
RD
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector). The ALE pulse in period T4 is only present if the next instruction
accesses the RAM (internal or external).
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