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bxbxb3
Guest
about clock doubling, one of my friend neil says:
"No really easy way, look at the specification of your device. Most of the
bigger have DLLs. if your have, use it.
Way back, when no devices had DLLs, we used a rather crude trick. You feed
the clock to one toggling DFF and also to another DFF via a lot of
instantiated inverters/buffers (to get a lot of delay). XOR the outputs
and you will get a pulse at the clock edge (the width will be the amount
of delay you have been able to introduce). Then we made another copy of
the logic, but with an inverted clock. XORing together the resulting
signals gave a short pulse at both rising and falling clock edge.That
meant we had actually doubled the clock frequency, but usually it
was of little use, since the clock was not nearly symmetrical. The fix for
that was to do it all over again and divide the resulting clock by two to
get it symmetrical.
Worked only for relatively slow clocks since the process used had to be
able to cope with twice the frequency we needed.I do not recommend you to
try this, it is just that I have filed it under the tab "meaningless
knowledge" and I do not seem to be able to scrub it (the tab) completeatly
clean. But, if you can not find another help you can try it.
When we used it, synthesis tools where not so advanced so they usually did
what they where told. Nowadays I guess it would be a lot of problems (if
possible) to get any of them not to optimize away the extra delay
introduced."
"No really easy way, look at the specification of your device. Most of the
bigger have DLLs. if your have, use it.
Way back, when no devices had DLLs, we used a rather crude trick. You feed
the clock to one toggling DFF and also to another DFF via a lot of
instantiated inverters/buffers (to get a lot of delay). XOR the outputs
and you will get a pulse at the clock edge (the width will be the amount
of delay you have been able to introduce). Then we made another copy of
the logic, but with an inverted clock. XORing together the resulting
signals gave a short pulse at both rising and falling clock edge.That
meant we had actually doubled the clock frequency, but usually it
was of little use, since the clock was not nearly symmetrical. The fix for
that was to do it all over again and divide the resulting clock by two to
get it symmetrical.
Worked only for relatively slow clocks since the process used had to be
able to cope with twice the frequency we needed.I do not recommend you to
try this, it is just that I have filed it under the tab "meaningless
knowledge" and I do not seem to be able to scrub it (the tab) completeatly
clean. But, if you can not find another help you can try it.
When we used it, synthesis tools where not so advanced so they usually did
what they where told. Nowadays I guess it would be a lot of problems (if
possible) to get any of them not to optimize away the extra delay
introduced."