Typewriter User Manual

10-8 MC68340 USER’S MANUAL MOTOROLA
Additionally, the relationship between the asynchronous inputs and the clock edge, as
shown in Figure 10-11, does not change as frequency changes.
A second type of specification indicates the minimum amount of time a signal will be
asserted. This type of specification is illustrated in Figure 10-12.
OUTPUT
CLKOUT
t
w
N
T/2
t
d
Figure 10-12. Signal Width Specifications
The method for calculating a frequency-adjusted t
w
is as follows:
t
w
' = t
w
+ N (T
f
'/2 – T
f
/2) + (T
f
'/2 – t
d
)
where:
t
w
' = the frequency-adjusted signal width
t
w
= the signal width at 16.78 MHz
N = the number of full one-half clock periods in t
w
T
f
'/2 = one-half the new clock period
T
f
/2 = one-half the clock period at full speed
t
d
= the propagation time from the clock edge
The following calculation uses a 16.78-MHz part, specification 14,
AS width asserted, at
12.5 MHz as an example:
t
w
= 100 ns
N = 3
T
f
'/2 = 80/2 = 40 ns
T
f
/2 = 60/2 = 30 ns
t
d
= 30 ns maximum
therefore:
t
w
' = 100 + 3(40 – 30) + (40 – 30) = 140 ns
The third type of specification used is a skew between two outputs (see Figure 10-13).
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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