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Crystal

A quartz crystal sets an accurate clock frequency for a chip, with two small load capacitors to ground. Modules like the ESP32 already have one inside.

What it does#

A quartz crystal resonates at one precise frequency. The chip’s oscillator pins (XIN/XOUT) drive it, and the crystal makes the oscillation exact enough for USB timing and serial ports. The catalog’s 12 MHz parts are what a bare RP2040 needs. Modules (ESP32-WROOM, XIAO, Pico) include their crystal: you only meet this part when you use a bare chip.

Load capacitors#

The crystal’s datasheet gives a load capacitance CL. Each pin gets a capacitor to GND of about:

C = 2 × (CL − C_stray), with C_stray ≈ 3–5 pF.

The catalog has two 12 MHz crystals with different CL, so they need different capacitors:

Crystal CL Capacitors Use
Abracon ABM8-272-T3 (C20625731) 10 pF 15 pF, plus 1 kΩ in XOUT bare RP2040, as in Raspberry Pi’s design guide
YXC X322512MSB4SI (C9002) 20 pF 33 pF other chips; basic part

Both numbers are right, for different crystals. Raspberry Pi’s guide uses the ABM8-272-T3 (CL = 10 pF): two 15 pF in series are 7.5 pF, plus about 3 pF of pins and traces gives 10.5 pF. C9002 is specified for 20 pF: 2 × (20 − 4) = 32 pF → 33 pF C0G. Swapping the capacitors between the two crystals pulls the frequency off and can stop the oscillator starting.

Example circuit: The 20 pF crystal (C9002) between XIN and XOUT, 33 pF C0G from each side to GND; the case pins (2, 4) to GND.
The 20 pF crystal (C9002) between XIN and XOUT, 33 pF C0G from each side to GND; the case pins (2, 4) to GND. Open the crystal_12mhz example from this page in PCB Canvas: “Insert into my schematic”.

The RP2040 crystal circuit#

The RP2040’s oscillator drives the crystal hard at 3.3 V, so Raspberry Pi puts 1 kΩ in series with XOUT. Together with the crystal’s maximum ESR of 50 Ω this keeps the drive level safe. The guide asks for exactly this crystal and circuit; any other crystal needs testing over temperature.

Example circuit: ABM8-272-T3 with 15 pF on each side; XOUT reaches the crystal through 1 kΩ, XIN directly.
ABM8-272-T3 with 15 pF on each side; XOUT reaches the crystal through 1 kΩ, XIN directly. Open the rp2040_crystal example from this page in PCB Canvas: “Insert into my schematic”.

Pitfalls#

  • Wrong load caps shift the frequency or stop the oscillator starting.
  • Layout: crystal and capacitors right next to the chip’s pins, short traces, ground under them, no signals routed between.
  • A second source for the ABM8-272-T3 is not drop-in: YXC X322512MMB4SI (C50430) also has CL = 10 pF but an ESR of 80 Ω, and the guide warns that a higher ESR lowers the amplitude with the 1 kΩ resistor.

Footprint#

3.2 × 2.5 mm 4-pad SMD (C9002 basic, C20625731 extended). Hot air or a fine iron.