Latch

XIAO RP2040 push-button power latch

This circuit powers a Seeed Studio XIAO RP2040 for as long as its button is held, then lets firmware keep power on for one second after release. After that the XIAO disconnects its own supply. Off-state current is principally MOSFET, diode, and capacitor leakage.

The populated circuit is in latch.sch. On first opening it, KiCad 10 will offer to convert the legacy container to latch.kicad_sch; save it beside the project files. The electrical content is not legacy-specific.

Connections

Q1 is the high-side switch. SW1 and D1 turn Q2 on before the XIAO has power. Firmware immediately drives POWER_HOLD high through D2, allowing Q2 to keep Q1 on after SW1 opens. Q3 provides a separately isolated button indication; this is necessary because Q2 holding the PMOS gate low otherwise looks exactly like a held button.

Use an AO3401A or another P-channel MOSFET specified for low resistance at VGS=-2.5 V. Do not substitute a MOSFET whose resistance is specified only at -10 V. Q2 and Q3 may be 2N7002 devices. D1 and D2 are low-leakage Schottky diodes; BAT54H is preferred over very leaky power Schottky parts.

The XIAO is intentionally fed through its 3V3 pin, not 5V, so its regulator is bypassed. Do not connect USB while the CR2032 is installed unless the power paths have first been checked for back-feeding on the exact XIAO revision.

CR2032 and coil warning

A CR2032 has appreciable internal resistance. C1 (100 uF) and C2 (100 nF) help with short load steps, but verify SYS_3V3 on an oscilloscope during RP2040 boot and coil pulses. Keep the coil off the GPIO: use a separate transistor driver and an appropriate inductive clamp. Size the battery and capacitor for peak current, not average current.

Firmware

The example uses the Raspberry Pi Pico SDK. Provide pico_sdk_import.cmake (normally copied or linked from the SDK), then build in the usual way:

cmake -S . -B build -DPICO_BOARD=seeed_xiao_rp2040
cmake --build build

power_latch_init() must remain the first initialization in main(). A press during the one-second release interval cancels shutdown.