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RP2040 chip

Raspberry Pi's dual Cortex-M0+ microcontroller as a bare QFN-56 chip. It needs QSPI flash, a 12 MHz crystal, 3.3 V and a handful of capacitors; the 1.1 V core supply is built in.

What it does#

The chip on the Raspberry Pi Pico: two Cortex-M0+ cores at 133 MHz, 264 KB RAM, 30 GPIOs (4 of them ADC inputs), USB 1.1 device or host and PIO state machines. It has no flash inside: the program lives in an external QSPI flash. A bare RP2040 board costs less than a module and fits your own shape, but you design the support circuit yourself.

What it needs#

  • 3.3 V on IOVDD (pins 1, 10, 22, 33, 42, 49), USB_VDD (48), ADC_AVDD (43) and VREG_VIN (44); 100 nF per IOVDD pin, right at it.
  • Core 1.1 V from the built-in regulator: VREG_VOUT (45) wired to both DVDD pins (23, 50), 1 µF at VREG_VIN and at VREG_VOUT, 100 nF at DVDD.
  • QSPI flash on the six QSPI pins (51-56), short and direct.
  • 12 MHz crystal on XIN/XOUT (20, 21) with the guide’s circuit (see Crystal).
  • USB: USB_DP (47) and USB_DM (46) through 27 Ω each, close to the chip; the pull-ups are inside.
  • TESTEN (19) to GND, the exposed pad (57) to GND with vias.
  • RUN (26): the reset input has an internal pull-up; a button to GND resets the chip.
  • BOOTSEL: a button that pulls QSPI_SS to GND through 1 kΩ while the chip starts makes it boot as a USB drive (drag a .uf2 file onto it).
Example circuit: The crystal circuit from Raspberry Pi's design guide: ABM8-272-T3, 15 pF on each side, 1 kΩ from XOUT.
The crystal circuit from Raspberry Pi's design guide: ABM8-272-T3, 15 pF on each side, 1 kΩ from XOUT. Open the rp2040_crystal example from this page in PCB Canvas: “Insert into my schematic”.

Layout#

  • 0.4 mm pitch: the pads are 0.2 mm wide with 0.2 mm between them. Tracks of 0.127 mm with 0.127 mm clearance (JLCPCB’s standard 2-layer rule) let every pad escape; wider supply tracks need a narrower class near the chip (0.25 mm is plenty for 3.3 V).
  • Put decoupling capacitors in the corners of the chip and leave a channel of about 2 mm along each pin row, so the signals can get out.
  • The exposed pad is fenced in by the pins: stitch it to the bottom GND pour with vias inside the pad.
  • Hand soldering a QFN-56 is hard; let JLCPCB assemble it.

Pitfalls#

  • No flash, no boot: with an empty or missing flash the chip only shows up as the BOOTSEL USB drive.
  • Wrong crystal capacitors (20 pF parts with 15 pF caps, or the reverse) give USB that works on some computers and not others.
  • DVDD is 1.1 V: never connect it to 3.3 V.

Sources: Raspberry Pi Hardware design with RP2040 (minimal design example) and RP2040 Datasheet (pin locations and descriptions).

Footprint#

QFN-56 7 × 7 mm, 0.4 mm pitch, 3.2 mm exposed pad (C2040, extended part). Machine assembly recommended.