Typewriter User Manual

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