Access Time Calculations - Motorola MC68020 User Manual

9.6 ACCESS TIME CALCULATIONS

The timing paths that are critical in any memory interface are illustrated and defined in
Figure 9-9.
The type of device that is interfaced to the MC68020/EC020 determines exactly which of
the paths is most critical. The address-to-data paths are typically the critical paths for
static devices since there is no penalty for initiating a cycle to these devices and later
validating that access with the appropriate bus control signal. Conversely, the address-
strobe-to-data-valid path is often most critical for dynamic devices since the cycle must be
validated before an access can be initiated. For devices that signal termination of a bus
cycle before data is validated (e.g., error detection and correction hardware and some
external caches), to improve performance, the critical path may be from the address or
strobes to the assertion of BERR (or BERR and HALT ). Finally, the address-valid-to-
DSACK1/DSACK0- asserted path is most critical for very fast devices and external
caches, since the time available between the address becoming valid and the
DSACK1/DSACK0 assertion to terminate the bus cycle is minimal. Table 9-4 provides
the equations required to calculate the various memory access times assuming a 50-
percent duty cycle clock.
CLK
A31–A0
*
AS
DSACK1/DSACK0
BERR, HALT
D31–D0
* For the MC68EC020, A23–A0.
NOTE: This diagram illustrates access time calculations only
Parameter
a
b
c
d
e
f
9-12
S0 S1 S2
a
Description
Address Valid to DSACK1/DSACK0 Asserted
AS Asserted to DSACK1/DSACK0 Asserted
Address Valid to BERR/HALT Asserted
AS Asserted to BERR/HALT Asserted
Address Valid to Data Valid
AS Asserted to Data Valid
M68020 USER'S MANUAL
c
b
d
System
t AVDL
t SADL
t AVBHL
t SABHL
t AVDV
t SADV
S0
e
f
Equation
9-3
9-4
9-5
9-6
9-7
9-8
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