
Applications Information
12-16
MC68030 USER’S MANUAL
MOTOROLA
Another way to optimize the CPU to memory access times in a system is to use a clock
frequency less than the rated maximum of the specific MC68030 device. Table 12-3
provides calculated t
AVDV
(see Equation 12-7 of Table 12-2) results for an MC68030RC16
and MC68030RC20 operating at various clock frequencies. If the system uses other clock
frequencies, the above equations can be used to calculate the exact access times.
12.4.2 Burst Mode Cycles
The memory access times for burst mode bus cycles follow the above equations for the first
access only. For the subsequent (second, third, and fourth) accesses, the memory access
time calculations depend on the architecture of the burst mode memory system.
Architectural tradeoffs include the width of the burst memory and the type of memory used.
If the memory is 128 bits wide, the subsequent operand accesses do not affect the critical
timing paths. For example, if a 3-1-1-1 burst accesses 128-bit-wide memory, the first access
is governed by the equations in Table 12-2 for N equal to three. The subsequent accesses
also use these values as a base but have additional clock periods added in. The second
access has one additional clock period, the third access has two additional clock periods,
and the fourth has three additional clock periods. Thus, the access time for the first cycle
determines the critical timing paths.
Table 12-3. Calculated t
AVDV
Values for Operation at Frequencies
Less Than or Equal to the CPU Maximum Frequency Rating
Equation 12-7
t
AVDV
Clocks Per BusCycle (N) and
Type
MC68030RC20
Clock at
16.67 MHz
MC68030RC25
Clock at
20 MHz
Wait
States
Clock at
20 MHz
Clock at
16.67 MHz
Clock at
25 MHz
38
—
78
78
118
118
158
158
198
198
2 Clock Synchronous
0
61
46
68
53
3 Clock Synchronous
3 Clock Asynchronous
4 Clock Synchronous
4 Clock Asynchronous
5 Clock Synchronous
5 Clock Asynchronous
6 Clock Synchronous
6 Clock Asynchronous
1
0
2
1
3
2
4
3
121
121
181
181
241
241
301
301
96
96
146
146
196
196
246
246
128
128
188
188
248
248
308
308
103
103
153
153
203
203
253
253