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Memory Controller
MOTOROLA
MPC801 USER’S MANUAL
15-37
15
15.2.3.10.1 Variable Access Time Solution.
on the local bus that addresses the main storage connected to the system bus. The
hierarchical bus interface accepts the local bus request and generates a read cycle on the
system bus. The programmer cannot foresee when the data will be valid to be latched by
the core since the system bus can be occupied by the DMA. There are two possible
solutions:
Assume that the core initiates a read cycle
The external module signals to the memory controller that the data is not ready yet by
asserting the UPWAIT signal. The memory controller synchronizes the signal since the
wait signal is asynchronous. When the wait signal is asserted, the user-programmable
machine enters freeze mode at the falling edge of the CLOCKOUT when the WAEN bit
is set in the UPM word. The user-programmable machine remains in this state as long
as the UPWAIT signal is asserted. After UPWAIT is negated, the user-programmable
machine will continue executing from the next entry to the end of the pattern and the
LAST bit is set.
The bus interface module signals to the memory controller when it can sample the data
by asserting the synchronous TA signal.
15.2.3.10.2 Slow Device Solution.
devices whose access time is greater than the maximum allowed by the user-programmable
machine. There are two possible solutions:
Assume that the core initiates a read cycle from slow
The core generates a read access from the slow device. The device will react by
asserting a wait signal as long as the data is not ready. The core will sample the data
only after the wait signal is negated.
The core generates a read access from the slow device, which is responsible for
generating the synchronous TZ when the data is ready.
15.2.3.10.3 Internal and External Synchronous Master.
WAEN bit in the word is read by the user-programmable machine and the UPWAIT signal
is used to hold the user-programmable machine in a particular state until the UPWAIT signal
is negated.
Figure 15-31 illustrates how the
The UPWAIT signal is sampled at the falling edge of the CLKOUT. If the signal is asserted
and the WAEN bit in the current user-programmable machine is enabled word, the user-
programmable machine is frozen until the UPWAIT signal is negated. The value of the
external pins driven by the user-programmable machine remains as indicated in the word
previously read by the user-programmable machine. When the UPWAIT signal is negated,
the user-programmable machine continues with its normal functions. During the wait cycles,
the TA signal is negated by the user-programmable machine.