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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 tLLRL+ tRLRH - tDVRH
in Tables 26-7 through Tables 26-14). 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 individ-
ual 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 26-7 through Tables 26-14 and Figure 26-6 to Figure 26-9 in the “External
Data Memory Characteristics” on page 374.
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 4-6.
External Data Memory Cycles no Wait-state (SRWn1=0 and SRWn0=0)
(1)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).
Figure 4-7.
External Data Memory Cycles with SRWn1 = 0 and SRWn0 = 1
(1)ALE
T1
T2
T3
Wr
ite
Read
WR
T4
A15:8
Address
Prev. addr.
DA7:0
Address
Data
Prev. data
XX
RD
DA7:0 (XMBK = 0)
Data
Prev. data
Address
Data
Prev. data
Address
DA7:0 (XMBK = 1)
System Clock (CLKCPU)
XXXXX
XXXXXXXX
ALE
T1
T2
T3
Wr
ite
Read
WR
T5
A15:8
Address
Prev. addr.
DA7:0
Address
Data
Prev. data
XX
RD
DA7:0 (XMBK = 0)
Data
Prev. data
Address
Data
Prev. data
Address
DA7:0 (XMBK = 1)
System Clock (CLKCPU)
T4