xiao-ble-sense-low-power

name	xiao-ble-sense-low-power
description	Domain knowledge for power optimization of the Seeed XIAO BLE Sense (nRF52840) under Zephyr NCS. Covers System OFF, QSPI Flash Deep Power-Down (P25Q16H), GPIO bitbang DPD, hardware floor, Device Tree overlays, and PPK2 measurement. Use this skill for: low-power optimization; System OFF implementation; QSPI flash power-down; GPIO hog configuration; current measurement with nRF PPK2; XIAO BLE Sense pin mapping; µA measurements; battery lifetime estimation.
user-invocable	true

XIAO BLE Sense — Low-Power Optimization (Zephyr NCS)

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1. Hardware Basics

Board: Seeed Studio XIAO BLE Sense

Property	Value
SoC	Nordic nRF52840
Flash (external)	Puya P25Q16H (128 Mbit QSPI)
Bootloader	Adafruit nRF52840 UF2 (0xADA52840)
App address	0x27000
Zephyr board name	xiao_ble_sense

QSPI Flash Pin Mapping (P25Q16H)

CRITICAL: This mapping deviates from intuitive expectations.

Signal	Pin	Direction	Note
QSPI_CSN	P0.25	OUT	Active-LOW (chip select)
QSPI_SCK	P0.21	OUT	Clock
QSPI_IO0 / MOSI	P0.20	OUT	SPI data (bitbang: MOSI)
QSPI_IO1 / MISO	P0.24	IN	SPI data (bitbang: MISO, not needed for DPD)
QSPI_IO2	P0.22	—	Quad mode (not needed for DPD)
QSPI_IO3	P0.23	—	Quad mode (not needed for DPD)

Source: xiao_ble-pinctrl.dtsi — NRF_PSEL(QSPI_CSN, 0, 25)

Hardware Floor (unavoidable quiescent current without HW modification)

The XIAO BLE Sense has external consumers that cannot be disabled in software:

• Charge IC (Seeed onboard): ~20–30 µA
• RGB LED pull-ups / passive circuitry
• LDO / DC-DC regulator

Measured hardware floor: ~44.7 µA (System OFF + flash DPD + IMU/MIC disabled via GPIO)

The nRF52840 itself draws < 2 µA in System OFF. The remaining 42+ µA come from the XIAO hardware — only reducible by hardware modification.

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2. Known Current Traps — Root Causes

🔴 Trap 1: GPIO Hog P0.25 OUTPUT-LOW → ~600 µA continuous current

Problem: P0.25 = QSPI_CSN (active-LOW). A GPIO hog with output-low on P0.25 holds the chip select permanently active → P25Q16H stays selected → ~600 µA drain.

Wrong (never use):

/* ❌ WRONG — keeps flash permanently selected */
&gpio0 {
    qspi-csn-low {
        gpio-hog;
        gpios = <25 GPIO_ACTIVE_HIGH>;
        output-low;
    };
};

Correct: Never hog P0.25. Power down flash via DPD command (→ Section 4).

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🔴 Trap 2: CONFIG_NORDIC_QSPI_NOR=y → 9.5 mA continuous current

Problem: The Zephyr driver nordic_qspi-nor performs JEDEC ID read and SFDP read sequences during initialization. This requires the HFXO (High Frequency Crystal Oscillator). The HFXO stays active → sys_poweroff() is blocked or never reached → SoC runs at full clock speed → 9.5 mA continuous current.

Wrong:

# ❌ Causes 9.5 mA, sys_poweroff() is never reached
CONFIG_NORDIC_QSPI_NOR=y

Correct: Disable the QSPI driver entirely, send DPD via GPIO bitbang (→ Section 4).

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🔴 Trap 3: CONFIG_PM_DEVICE_RUNTIME=y → 10 mA / crash loop

Problem: PM_DEVICE_RUNTIME creates work queue threads. With a small stack (CONFIG_MAIN_STACK_SIZE=1024) → stack overflow → crash loop → high continuous current.

Correct: Omit PM_DEVICE_RUNTIME. No PM framework needed for a one-shot power-down.

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🟡 Trap 4: IMU and microphone power supply

Consumer	Pin	Action
LSM6DS3TR-C IMU VCC	P1.08	GPIO hog OUTPUT-LOW
MSM261D3526H1 Microphone VCC	P1.10	GPIO hog OUTPUT-LOW

/* ✅ Correct: disable IMU and MIC via hog */
&gpio1 {
    pwr-mic {
        gpio-hog;
        gpios = <10 GPIO_ACTIVE_HIGH>;
        output-low;
        line-name = "MIC Power";
    };
    pwr-imu {
        gpio-hog;
        gpios = <8 GPIO_ACTIVE_HIGH>;
        output-low;
        line-name = "IMU Power";
    };
};

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3. Reference Configuration (Sample 099)

prj.conf — minimal base

CONFIG_GPIO=y
CONFIG_TICKLESS_KERNEL=y
CONFIG_MAIN_STACK_SIZE=1024
CONFIG_IDLE_STACK_SIZE=512

boards/xiao_ble_sense.conf

# RC oscillator instead of XTAL for the 32 kHz clock (saves ~200 µA in sleep)
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

# Disable console, USB, logging, and sensor drivers
CONFIG_USB_DEVICE_STACK=n
CONFIG_USB_UART_CONSOLE=n
CONFIG_SERIAL=n
CONFIG_CONSOLE=n
CONFIG_UART_CONSOLE=n
CONFIG_LOG=n
CONFIG_SENSOR=n
CONFIG_LSM6DSL=n

# Release NFC pins as GPIOs (otherwise P0.09/P0.10 are reserved as NFC antenna)
CONFIG_NFCT_PINS_AS_GPIOS=y

boards/xiao_ble_sense.overlay — relevant QSPI section

/* ── Disable QSPI peripheral ────────────────────────────────────────────
 * The Zephyr nordic_qspi-nor driver is NOT used.
 * Deep Power-Down (0xB9) is sent via GPIO bitbang in main.c.
 * WARNING: P0.25 = QSPI_CSN (active-LOW) — never hog OUTPUT-LOW!
 */
&qspi { status = "disabled"; };

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4. P25Q16H Flash — Deep Power-Down via GPIO Bitbang

This is the only reliable way to put the flash into DPD before sys_poweroff() without activating the Zephyr QSPI driver (which would otherwise hold the HFXO).

Background

• DPD opcode: 0xB9
• t_enter_dpd: 3 µs (from DT: t-enter-dpd = <3000> ns)
• t_exit_dpd: 8 ms (from DT: t-exit-dpd = <8000> ns)
• SPI mode: Mode 0 (CPOL=0, CPHA=0), MSB-first
• Protocol: CS low → 8 bits → CS high → wait t_enter_dpd

C Implementation (direct nRF HAL usage)

#include <hal/nrf_gpio.h>

/* P25Q16H QSPI pins on XIAO BLE Sense */
#define FLASH_PIN_MOSI  20u   /* P0.20 = QSPI_IO0 */
#define FLASH_PIN_SCK   21u   /* P0.21 = QSPI_SCK */
#define FLASH_PIN_CSN   25u   /* P0.25 = QSPI_CSN (active-LOW!) */
#define FLASH_CMD_DPD   0xB9u

static void flash_deep_power_down(void)
{
    /* Configure pins as outputs */
    nrf_gpio_cfg_output(FLASH_PIN_CSN);
    nrf_gpio_cfg_output(FLASH_PIN_SCK);
    nrf_gpio_cfg_output(FLASH_PIN_MOSI);

    /* Idle state: CSN HIGH (deselected), SCK LOW */
    nrf_gpio_pin_set(FLASH_PIN_CSN);
    nrf_gpio_pin_clear(FLASH_PIN_SCK);
    nrf_gpio_pin_clear(FLASH_PIN_MOSI);

    /* Assert CS (select flash) */
    nrf_gpio_pin_clear(FLASH_PIN_CSN);

    /* Send 0xB9 MSB-first via SPI Mode 0 */
    for (int i = 7; i >= 0; i--) {
        nrf_gpio_pin_write(FLASH_PIN_MOSI, (FLASH_CMD_DPD >> i) & 1u);
        nrf_gpio_pin_set(FLASH_PIN_SCK);    /* rising edge — data is latched */
        nrf_gpio_pin_clear(FLASH_PIN_SCK);  /* falling edge */
    }

    /* Deassert CS → DPD command complete */
    nrf_gpio_pin_set(FLASH_PIN_CSN);

    /* Wait t_enter_dpd = 3 µs (conservative: 10 µs) */
    k_busy_wait(10u);
}

Call in main.c (before sys_poweroff()):

flash_deep_power_down();
sys_poweroff();

No include overhead: <hal/nrf_gpio.h> is always available, requires no Kconfig symbol.

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5. System OFF Checklist

Before calling sys_poweroff(), all consumers must be powered down:

• QSPI flash: DPD via bitbang (Section 4)
• IMU LSM6DS3TR-C: GPIO hog P1.08 OUTPUT-LOW
• Microphone MSM261D3526H1: GPIO hog P1.10 OUTPUT-LOW
• QSPI driver: &qspi { status = "disabled"; } in overlay
• CONFIG_NORDIC_QSPI_NOR: must not be set
• CONFIG_PM_DEVICE_RUNTIME: must not be set
• CONFIG_NFCT_PINS_AS_GPIOS=y: set (otherwise P0.09/P0.10 are used as NFC)
• 32 kHz clock: CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

Expected result: ~44.7 µA (hardware floor of the XIAO BLE Sense)

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6. PPK2 Measurement and Analysis

Measurement

• Mode: Source Mode, 3.3 V
• Connection: V+ → XIAO 3V3 pin, GND → XIAO GND
• Software: nRF Power Profiler (Desktop) or nRF PPK2 app
• D0 digital input: GPIO of the nRF52840 (e.g. P0.05 = D5) for timestamps

Analysis with the analysis script

uv run scripts/ppk_analysis.py data/<file>.ppk2

The script provides:

• Total duration, sample rate, mean, peak
• Boot phase (0–1010 ms) is automatically excluded
• Steady-state mean (after boot phase) = the relevant figure
• D0-HIGH phase analysis (timestamps, energy)
• Battery lifetime estimate for CR2032, 2× AA, 2× AAA

Reference measurement (Sample 099, GPIO bitbang DPD)

Metric	Value
Steady-state avg	44.70 µA
Boot peak	79.5 mA (short burst)
D0-HIGH duration	50 ms (boot marker)
D0-HIGH avg current	2338 µA
CR2032 (230 mAh)	~214 days
2× AA (2500 mAh)	~6.4 years

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7. Flash Workflow (XIAO BLE Sense)

# Build + flash (default port /dev/ttyACM3)
make 099-sysoff-serial-flash BOARD=xiao_ble_sense

# Specify a different port
make 099-sysoff-serial-flash BOARD=xiao_ble_sense SERIAL_PORT=/dev/ttyACM1

Prerequisite: Double-click RESET until the LED fades (DFU mode), then flash.

Flash Toolchain

• Tool: adafruit-nrfutil 0.5.3.post16
• Installation: uv tool install adafruit-nrfutil
• UF2 family: 0xADA52840
• App address: 0x27000

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8. Lessons Learned (Session Summary)

#	Finding
1	P0.25 on XIAO BLE Sense is QSPI_CSN (active-LOW), not a VCC enable
2	GPIO hog OUTPUT-LOW on P0.25 → flash always selected → ~600 µA
3	CONFIG_NORDIC_QSPI_NOR=y holds HFXO after init → sys_poweroff() unreachable → 9.5 mA
4	CONFIG_PM_DEVICE_RUNTIME=y → work queue + stack overflow → crash loop → ~10 mA
5	GPIO bitbang (direct nRF HAL) is more reliable than Zephyr PM framework for a one-shot power-down
6	This pattern matches exactly the Adafruit Arduino sketch (transport.runCommand(0xB9) before SYSTEMOFF)
7	Hardware floor of the XIAO BLE Sense: ~44.7 µA — only reducible by hardware modification