MC68020 Motorola, MC68020 Datasheet - Page 268

• Manufacturer Part Number: MC68020
• Description: (MC68020 / MC68EC020) MICROPROCESSORS USERS MANUAL
• Manufacturer: Motorola

During asynchronous bus cycles, DSACK1 / DSACK0 are used to terminate the current
bus cycle. In true asynchronous operations, such as accesses to peripherals operating at
a different clock frequency, either or both signals may be asserted without regard to the
clock, and then data must be valid a certain amount of time later as defined by
specification 31. With a 25-MHz controller, this time is 32 ns after DSACK1 / DSACK0
asserts; with a 16.67-MHz controller, this time is 50 ns after DSACK1 / DSACK0 asserts
(both numbers vary with the actual clock frequency).
However, many local memory systems do not operate in a truly asynchronous manner
because either the memory control logic can be related to the MC68020/EC020 clock or
worst-case propagation delays are known; thus, asynchronous setup times for the
DSACK1 / DSACK0 signals can be guaranteed. The timing requirements for this pseudo-
synchronous DSACK1 / DSACK0 generation is governed by the equation for t
9-14
Where:
Equation
Equation
t27A
t47A
t27
9-3
9-4
9-5
9-6
9-7
9-8
9-3
9-4
9-5
9-6
9-7
9-8
tX
t1
t2
t3
t6
t9
N = The Total Number of Clock Periods in the Bus Cycle (N
= Refers to AC Electrical Specification X
= The Clock Period
= The Clock Low Time
= The Clock High Time
= The Clock High to Address Valid Time
= The Clock Low to AS Low Delay
= The Data-In to Clock Low Setup Time
= The BERR / HALT to Clock Low Setup Time
= The Asynchronous Input Setup Time
Table 9-4. Memory Access Time Equations at 16.67 and 25 MHz
t
t
t
t
t
t
t
t
t
t
t
t
AVDL
SADL
AVBHL
SABHL
AVDV
SADV
AVDL
SADL
AVBHL
SABHL
AVDV
SADV
= (N – 1) • t1 – t2 – t6 – t47A
= (N – 1) • t1 – t9 – t60
= N • t1 – t2 – t6 – t27
= (N – 1) • t1 – t9 – t27
= (N – 1) • t1 – t2 – t6 – t47A
= (N – 1) • t1 – t9 – t60
= N • t1 – t2 – t6 – t27
= (N – 1) • t1 – t9 – t27
= N • t1 – t2 – t6 – t27A
= (N – 1) • t1 – t9 – t27A
= N • t1 – t2 – t6 – t27A
= (N – 1) • t1 – t9 – t27A
16.667 MHz
25 MHz
M68020 USER’S MANUAL
N = 3
N = 3
121
25
85
17
57
61
22
40
31
22
26
71
3 Cycles)
N = 4
N = 4
121
181
145
111
85
46
70
71
57
41
44
97
N = 5
N = 5
181
145
100
241
205
111
151
137
70
97
60
62
N = 6
N = 6
241
205
130
301
265
151
137
191
177
94
79
80
AVDL
N = 7
N = 7
301
265
118
160
361
325
191
177
231
217
98
98
MOTOROLA
.
Unit
Unit
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns