Note that U4, a 10 MHz MEMS oscillator, is also very close to the microcontroller’s clock input pin. It’s always a good idea to minimize the length of traces carrying high-frequency digital signals. First of all, there are noise benefits: a shorter, more direct trace reduces the amount of noise that would otherwise be coupled into adjacent traces, and a shorter trace also reduces electromagnetic interference (EMI) because it is less effective as an antenna. The second issue is related to transmission-line effects. Minimizing trace length is a simple way to avoid problems related to signal reflections. However, reflection is not a significant concern at frequencies in the 10 MHz range, unless you are dealing with long interconnections or a very large PCB.
It’s always good to be cognizant of trace lengths when you’re laying out a parallel bus, though at moderate frequencies it is nothing to stress about. The propagation time for a signal traveling through a trace is maybe 150 picoseconds/inch. So if you have two traces with a length mismatch of one inch, one signal will arrive 150 ps after the other signal. If your signals are transitioning at a frequency whose corresponding period is much greater than 150 ps, this one-inch mismatch won’t cause problems. Even at 100 MHz (which is pretty fast for a parallel bus), the period is 10 ns, i.e., ~67 times larger than the time-of-arrival discrepancy for a one-inch mismatch.
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