Sharp MZ-80B Owner's Manual page 84

77
INSTRUCTION FETCH
Figure 3.0-1 shows the timing during an Ml cycle (OP code fetch). Notice that the PC is placed on the address bus
at the beginning of the Ml cycle . One half clock time later the MREQ signal goes active. At this time the address to the
memory has had time to stabilize so that the falling edge of MREQ can be used directly as a chip enable clock to
dynamic memories. The RF line also goes active to indicate that the memory read data should be enabled onto the CPU
data bus. The CPU samples the data from the memory on the data bus with the rising edge of the clock of state T3 and
this same edge is used by the CPU to turn off the RD and MRQ signals. Thus the data has already been sampled by the
CPU before the RD signal becomes inactive. Clock state T3 and T4 of a fetch cycle are used to refresh dynamic memo-
ries. (The CPU uses this time to decode and execute the fetched instruction so that no other operation could be per-
formed at this time). During T3 and T4 the lower 7 bits of the address bus contain a memory refresh address and the
RFSH signal becomes active to indicate that a refresh read of all dynamic memories should be accomplished. Notice
that a RD sgianl is not generated during refresh time to prevent data from different memory segments from being gated
onto the data bus. The MREQ signal during refresh time should be used to perform a refresh read of all memory ele-
ments. The refresh signal can not be used by itself since the refresh address is only guaranteed to be stable during
MREQ time.
T ,
-
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y
PC
MREQ \
\
-
,_
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r - - - - - - -
-
1\
DBO DB?
M I Cycle
T z T 3 T 4
~ ~
I L -
X
REFRESH ADDR.
I
I
_-y-c_ - - - - - -
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INSTRUCTION OP CODE FETCH
FIGURE 3.0-1
T l
~
X
\
------
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_____
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I
Figure 3.0-lA illustrates how the fetch cycle is delayed if the memory activates the WAIT line. During T2 and
every subsequent Tw, the CPU samples the WAIT line with the falling edge of <I>. If
the WAIT line is active at this time,
another wait state will be entered during the following cycle . Using this technique the read cycle can be lengthened to
match the access time of any type of memory device .