Intel Embedded Intel486, Hardware Reference Manual

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EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
4-16
4.3.1 Non-Cacheable Non-Burst Single Cycle
4.3.1.1 No Wait States
The fastest non-burst bus cycle that the Intel486 processor supports is two clocks. These cycles
are called 2-2 cycles because reads and writes take two cycles each. The first “2” refers to reads
and the second “2” to writes. If a wait state needs to be added to the write, the cycle is called “2-
3.”
Basic two-clock read and write cycles are shown in Figure 4-10 . The Intel486 processor initiates
a cycle by asserting the address status signal (ADS#) at the rising edge of the first clock. The
ADS# output indicates that a valid bus cycle definition and address is available on the cycle def-
inition lines and address bus.
Figure 4-10. Basic 2-2 Bus Cycle
The non-burst ready input (RDY#) is asserted by the external system in the second clock. RDY#
indicates that the external system has presented valid data on the data pins in response to a read
or the external system has accepted data in response to a write.
The Intel486 processor samples RDY# at the end of the second clock. The cycle is complete if
RDY# is asserted (LOW) when sampled. Note that RDY# is ignored at the end of the first clock
of the bus cycle.
The burst last signal (BLAST#) is asserted (LOW) by the Intel486 processor during the second
clock of the first cycle in all bus transfers illustrated in
Figure 4-10 . This indicates that each trans-
CLK
ADS#
A31–A2
M/IO#
D/C#
BE3#–BE0#
W/R#
RDY#
BLAST#
DATA
PCHK#
Ti T1 T2 T1 T2 T1 T2 T1 T2 Ti
Read Write Read Write
To Processor
From Processor
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