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will not delay for one clock tick! That would be impossible! There is no event exactly one clock tick after usleep() is called; the next timer tick will always occur at some time strictly less than the system timer interval. Rather, usleep() will delay for two clock ticks resulting some actual delay between 10 and 20 microseconds, exclusive. See the following figure:

http://nuttx.org/Images/short-delay.pngImage RemovedImage Added

For the most part, when delays are large – hundreds or thousands of microseconds – this error is not significant. It becomes noticeable only if you are using usleep() for delays very close to the the timer resolution when the error can be relatively significant.

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