Now to make our USB and other peripherals actually work properly, we need to have what's called a crystal oscillator. This is a little piezoelectric quartz crystal that vibrates very precisely, and then it's amplified and fed into the MCU to act as a clock signal that controls the digital peripherals.
For example, you definitely want a crystal oscillator if you're using USB-C, because the data needs to come in at specific times, so it makes sure no data is incorrectly received. Because the clock is such a precise component, you want to wire it really carefully. That means it should be as close to the MCU as possible on the PCB (schematic doesn't matter, it's just a reference), and it needs really small capacitors, to smooth out the signals.
#Label XIN and XOUT
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First add the global labels to the MCU XIN and XOUT, just called their relative name. XOUT is the output from the crystal so an input to the MCU, and XIN is an output from the MCU to help the crystal oscillate properly:
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Remember to accurately represent your global label direction, but just keep in mind it doesn't actually change your schematic, it's only for whoever is reading it!
#Place the crystal symbol
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Based off the RP2040 datasheet, we're going to be using a 12 MHz crystal with two, 15pF (I switch these later to 33pF because we use a different crystal than the Pi Pico) decoupling capacitors.
#Wire the crystal
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Pins 2, and 4 just go to GND, pins 1 and 3 need a 15pF cap in series, and XOUT will have a 1K resistor. This resistor is called a damping resistor and it prevents the crystal from being damaged and ensures good signal integrity:
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Remember all your schematic good practices and make sure everything looks clean.
We haven't actually seen these types of caps yet, these are called external load capacitors, and they're placed in series with the crystal I/O's, these basically just ensure that the crystal resonates at it's proper frequency, I'd suggest researching a bit more if you're interested!