sparkfun Allband GNSS RTK Breakout - ZED-X20P (Qwiic)

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Dense context for pasting into a model prompt — addresses, pinout, voltages, tolerances and the vendor's reference circuit.

PART: SparkFun Allband GNSS RTK Breakout - ZED-X20P (Qwiic), GPS-28871, 43.2 mm x 43.2 mm (1.70 in square) with four 4-40 mounting holes, built around a u-blox ZED-X20P-01B all-band GNSS module (17.0 mm x 22.0 mm x 2.4 mm). The board adds a USB-C receptacle, an RT9080-3.3 3.3 V regulator, 19 through-hole pads, two Qwiic sockets, a 6-pad BlueSMiRF header and a 4-pad locking JST (both on UART2), an SMA connector with a U.FL alternative behind a jumper, a rechargeable backup battery, and PWR/PPS/RTK/FENCE LEDs. Constellations: GPS L1C/A L2C L5, Galileo E1-B/C E5a E6, BeiDou B1I B1C B2a B3I, QZSS L1C/A L1C/B L2C L5, NavIC L1 L5, SBAS on L1C/A, plus L-band corrections handled on the module itself.
I2C: 7-bit address 0x42, peripheral only, standard/fast/fast-mode-plus up to 1000 kbit/s, clock stretching up to 20 ms, 400 pF per bus line (550 pF in fast-mode-plus). There is no address strap on the board, so two receivers on one bus must be given different addresses in software.
PINOUT: J3 (1x09) pad 1 SCL, 2 TX1, 3 RX1, 4 SDA, 5 RX2, 6 TX2, 7 3V3, 8 VIN, 9 GND. J11 (1x06) pad 1 FENCE, 2 RTK, 3 PPS, 4 RST, 5 EN, 6 EVENT. J9 (1x04, I2C) pad 1 GND, 2 3V3, 3 SDA, 4 SCL. J2 (1x06, BlueSMiRF) pad 1 NC, 2 TXD, 3 RXD, 4 3V3, 5 NC, 6 GND. J8 (1x04, locking JST) pad 1 3V3, 2 TXD, 3 RXD, 4 GND. Both Qwiic sockets are pad 1 GND, 2 3V3, 3 SDA, 4 SCL. On J3 the I2C names sit on pads 1 and 4 while the same pins are SPI clock and SPI chip select when D_SEL is low; RX1 and TX1 become SDI and SDO.
SUPPLY: VIN takes 1.2-5.5 V and the RT9080-3.3 makes the 3.3 V rail at up to 600 mA; SparkFun recommends at least 3.5 V at VIN for a clean 3.3 V output. The module itself runs 2.7-3.6 V (3.3 V typical). Draw at 3.3 V is 68 mA acquiring and 64 mA tracking on GPS+Galileo+BeiDou, 55 mA on GPS alone, with peaks to 85 mA during acquisition - size the feed for that. 5 V is only needed for USB, and only when the VSEL jumper selects it does the JST's VCC pin carry 5 V.
LOGIC: 3.3 V, all PIOs referenced to VCC. Absolute maximum VCC -0.3 to 3.6 V, any digital IO pin -0.3 V to VCC+0.3 V with a 3.6 V ceiling, source or sink 10 mA per pin, RF_IN -5.5 to +5.5 V DC and +15 dBm; the module has no overvoltage or reverse-voltage protection, so a 5 V host will damage it. Internal pull-ups: 8-40 kOhm on SCL, SDA, RXD2, TXD2, TIMEPULSE, TX_READY and the antenna pins, 35-205 kOhm on D_SEL, RXD, TXD, SAFEBOOT_N, EXTINT, GEOFENCE_STAT and RTK_STAT, 7-13 kOhm on RESET_N. Unused inputs can be left open.
REFERENCE CIRCUIT: 3.3 V carries C2 10 uF, C5 0.1 uF and C8 33 pF; V_BCKP has C9 1 uF plus the backup cell; the antenna rail VCC_RF is fed through R6 10 Ohm with C3 0.1 uF to ground. I2C pull-ups R12 (SCL) and R13 (SDA) are 2.2 kOhm and reach 3.3 V only through JP9, a 3-pole solder jumper that ships open - bridge pad 1-2 and pad 2-3 to put them on the bus. J2/J8 RXD passes through R8 1k. Each status LED has its own cuttable jumper (PWR, PPS, RTK, FENCE).
TRAPS: RST held LOW for 100 ms or more is a cold start that erases configuration, RTC and orbit data, and a capacitor from RST to GND re-triggers a reset on every power-up, so fit none. SAFEBOOT_N and TIMEPULSE are joined inside the module by a 1 kOhm series resistor, so anything loading the PPS pad low at startup drops the receiver into safeboot mode. Firmware updates only over UART1. The BlueSMiRF header and the JST are the same UART2 - populate one, not both, or the two drivers contend. The UART receiver turns itself off when it counts more than 100 framing errors in one second, which is what a wrong baud rate or a grounded RX line produces. SPI is off until the D_SEL jumper is closed, and closing it removes I2C from those pins. The U.FL connector only works after the RF jumper moves the antenna path off the SMA. Defaults: UART1 NMEA at 38400 8N1, UART2 RTCM 3.4 out and RTCM/SPARTN in with NMEA disabled, UART2 also carries UBX.