MC68030 Datasheet, PDF (530/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Applications Information
To minimize the potential for delays in retrying a bus cycle, the negation path of the bus error
and halt signals should be carefully controlled. Light capacitive loading of these signals lines
as well as the use of a properly sized pullup resistor for any open collector drivers, or some
equivalent method, is recommended.
The available cache tag lookup, compare, and logic delay (D) and (E) time for this
implementation is given by Equation 12-5 of Table 12-2 (40 ns at 20.0 MHz with no wait
states).
A further design consideration is the response of the main memory controller to accesses
that miss in the cache and are retried. During a retry operation and in the absence of
arbitration for the logical bus, the MC68030 continuously drives the address bus with the
address that caused the retry to be signaled. This presents the designer with the opportunity
to utilize this information to continue (or initiate) the access in the main memory (by latching
the state of the AS signal during the initial bus cycle and holding it asserted for the duration
of the retry), thus decreasing the overhead associated with retrying the cycle.
12.6.2 Instruction-Only External Cache Implementations
In some cases, particularly in multiprocessing systems where cache coherence is a
concern, it is desirable to store only instruction operands since they are not considered to
be alterable and, hence, cannot generate stale data. In general, this is feasible with the
MC68000 architecture as long as PC relative addressing modes are not used. This
restriction allows program and data accesses to be distinguished externally by decoding the
function code signals.
12.7 DEBUGGING AIDS
The MC68030 supports the monitoring of internal microsequencer activity with the STATUS
and REFILL signals. The use of these signals is described in the following paragraph. A
useful device to aid programming debugging is described in 12.7.2 Real-Time Instruction
Trace.
12-34
MC68030 USER’S MANUAL
MOTOROLA