6
SY87729/39L
Evaluation Board
Micrel, Inc.
M9999-071906
[email protected] or (408) 955-1690
Figure 11. Desired Output Frequency Cannot Be
Synthesized
The error message is telling you that SY87729/39L cannot
synthesize an 8.9MHz clock from a 27MHz clock.
Remember, the smallest output frequency SY87729/39L can
generate is 9.0MHz, no matter what the input frequency is.
Let’s try another example. Suppose we want to generate
111,100,000MHz from 27MHz. Hit the “Tab” key until the
output frequency value is highlighted, and then type “1”,
“1”, “1”, “.”, “1”, and “m”. Your calculation module should
look like that in Figure 12.
Figure 12. Generating 111.1MHz With the Wrapper
The status line begins with the word SUCCESS. This
means that SY87729/39L can generate the output frequency
specified. The closest frequency it can generate is shown
in the “Actual” box. In this case, it is 111,099,130MHz, just
a bit low. The status line shows that the actual output
frequency differs from the desired (or target) output
frequency by 8ppm in frequency.
On the far right, the six SY87729/39L configuration
parameters to generate this output frequency are shown.
These parameters could be used by a firmware engineer to
program an embedded processor to configure
SY87729/39L.
In the above example, the wrapper synthesizer is used
to obtain the best possible actual output frequency. If the
wrapper synthesizer is not used, the calculation module
should look like that in Figure 13.
Figure 13. Generating 111.1MHz Without the Wrapper
Note that in this example, the “M” and “N” configuration
parameters are both 14. This means that the wrapper
synthesizer is passing the frequency of the fractional-N
synthesizer unchanged. Because of this, the nearest
frequency is now 111,103,448Hz, just a bit higher than the
target frequency. The status line indicates that the frequency
error is now 32ppm, which is not as good as the 8ppm error
when the wrapper synthesizer is used.
Let’s try another example. Let’s generate an OC-12 clock
from this 27MHz input frequency. The calculation module
should look like that shown in Figure 14. Note that, since
the wrapper synthesizer was reserved, both the “M” and the
“N” configuration parameters are set to 14.
Figure 14. Generating 622.08MHz Without the Wrapper
(Using SY87739L only)
Now let’s see what happens when you use the wrapper
synthesizer. Your calculation module should look like that
in Figure 15.