Freescale Semiconductor, Inc.
T/2
CLKOUT
t
d1
OUTPUT1
OUTPUT2
t
d2
t
s
Figure 10-13. Skew between Two Outputs
The method for calculating a frequency-adjusted t is as follows:
s
t ' = t + N (T '/2 – T /2) + (T '/2 – t )
d1
s
s
f
f
f
where:
t ' = the frequency-adjusted skew
s
t = the skew at full speed
s
N = the number of full one-half clock periods in t , if any
s
T '/2 = one-half the new clock period
f
T /2 = one-half the clock period at full speed
f
t
= the propagation time for the first output from the clock edge
d1
The following calculation uses a 16.78-MHz port, specification 21, R/W high to A S
asserted, at 8 MHz as an example:
t = 15 ns minimum
s
N = 0
T '/2 = 125/2 = 62.5 ns
f
T /2 = 60/2 = 30 ns
f
t
= 30 ns maximum
d1
therefore:
t ' = 15 + 0(62.5 – 30) + (62.5 – 30) = 47.5 ns minimum
s
In this manner, new specifications for lower frequencies can be derived for an MC68340.
MOTOROLA
MC68340 USER’S MANUAL
10-9
For More Information On This Product,
Go to: www.freescale.com
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