As you may have noticed, this is one of those annoying situations in which the pin configuration is the opposite of what you want: the DAC’s input pins move from bit 7 to bit 0 going downward, whereas the MCU’s output pins move from bit 7 to bit 0 going upward. I couldn’t remedy this by re-assigning the microcontroller pins because I wanted the data bits to correspond to the actual bits of the Port 2 register (so that I could write a full byte to the DAC without moving bits around). So I ended up with some awkward routing, but nothing terrible.
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.
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