pimoroni PCF8523 Pre-Assembled PiCowbell Adalogger for Pico - MicroSD, RTC & STEMMA QT

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

BOARD: Adafruit product 6355, the pre-assembled Adafruit PiCowbell Adalogger for Pico - the same 53 x 21 mm footprint as a Raspberry Pi Pico, with two 20-pin stacking headers (one element, CB1, pads 1-40) so it sockets straight underneath. On board: a microSD socket (Molex 104031-0811), an NXP PCF8523T real-time clock in SOIC-8 with a 32.768 kHz crystal (XTAL3215) and a CR1220 coin-cell backup holder, a right-angle JST-SH STEMMA QT / Qwiic socket, a reset button, and a row of duplicate hole pads beside most header pins for solder-jumpering. Adafruit is the maker; Pimoroni resells this SKU as ADA6355 at 10.00 GBP as measured.
ADDRESSES: the PCF8523 answers at I2C 0x68, fixed - Adafruit's pinouts page states it as the default address and the part has no address pins, so no other address is reachable without a multiplexer. Nothing else on the board is addressable: the microSD is SPI with its own chip select.
PINOUT: 40 pads on the header, numbered as the Pico's own pins 1-40 (CB1 pad 1 is the Pico's pin 1; the fab file's nets match Raspberry Pi's pinout pad for pad). Pad 1 GP0, 2 GP1, 3 G, 4 GP2, 5 GP3, 6 SDA (GP4), 7 SCL (GP5), 8 G, 9 GP6, 10 GP7, 11 GP8, 12 GP9, 13 G, 14 GP10, 15 GP11, 16 GP12, 17 GP13, 18 G, 19 GP14, 20 GP15 (SD card detect when the SD Det jumper is closed), 21 MI (SPI MISO, GP16), 22 CS (microSD chip select, GP17), 23 G, 24 SCK (GP18), 25 MO (SPI MOSI, GP19), 26 GP20, 27 GP21, 28 G, 29 GP22, 30 Reset (RUN, with the on-board button), 31 A0 (GP26), 32 A1 (GP27), 33 G (analog ground), 34 A2 (GP28), 35 VR (ADC_VREF), 36 3V, 37 EN (3V3_EN), 38 G, 39 VS (VSYS), 40 VB (VBUS). Labels are the words Adafruit's pinouts page says are printed on the silk; the fab file's nets agree (SDA, SCL, MISO, MOSI, SCK, CARD_CS, CARD_DET, 3.3V, VSYS, VBUS, !RESET, !EN, VREF, GND).
SUPPLY: the board takes 3.3 V from the host's 3V pin - it has no regulator of its own - and the PCF8523's clock supply is rated 1.0 V to 5.5 V, so the RTC itself is not the limit. RTC timekeeping continues from the CR1220 cell when host power is removed; the cell is not included and is required for battery backup.
LOGIC LEVEL: 3.3 V. SDA and SCL are pulled to 3.3 V by 10 kOhm resistors R1 and R2 fitted on the board, so the bus is 3.3 V logic only - a 5 V host must not drive these lines without level shifting, and the pull-ups cannot be removed except by desoldering them.
REFERENCE CIRCUIT: as built, from the fab file. R1 10 kOhm SDA to 3.3 V and R2 10 kOhm SCL to 3.3 V. The PCF8523's VDD (pin 8) is fed from 3.3 V through R3 1 kOhm with C1 10 uF to ground, and C2 10 uF sits on the 3.3 V rail. R4 10 kOhm pulls the microSD chip select (CARD_CS) up to 3.3 V so the card stays deselected while the host is booting. The 32.768 kHz crystal is Q1. Nothing needs adding: the pull-ups, decoupling and crystal load capacitors are all fitted, and the PCF8523's own oscillator load capacitors are internal and selected in software (7 pF default, 12.5 pF by setting CAP_SEL).
TRAPS: SDA and SCL are GPIO4 and GPIO5, and they are the same nets as the STEMMA QT socket, so anything plugged into the STEMMA port shares the bus with the RTC - two devices at 0x68 collide. The microSD is on GP16-GP19 (MI, CS, SCK, MO), which is not the default SPI0 pin set a Pico sketch might assume, so the SPI instance must be remapped. Card detect is NOT connected by default: the SD Det solder jumper on the back, directly above GP15 and labelled SD Det, must be bridged to bring the card-detect line to GP15, and with it open the card-detect pin floats. Adafruit ships no microSD card and no CR1220 cell. On the Philhower Arduino core Wire already points at GPIO4/5; on CircuitPython or MicroPython the pins must be set explicitly to 4 and 5. Adafruit's revision history says a square-wave breakout hole for the RTC was added on 28 October 2025, but the fab file in the PCB repo binds the SQW net only to the PCF8523's pin 7 with no connector, so it is not in this pin list.