| 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)
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.
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).
🔴 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).
🔴 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.
🟡 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";
};
};
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"; };
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>
#define FLASH_PIN_MOSI 20u
#define FLASH_PIN_SCK 21u
#define FLASH_PIN_CSN 25u
#define FLASH_CMD_DPD 0xB9u
static void flash_deep_power_down(void)
{
nrf_gpio_cfg_output(FLASH_PIN_CSN);
nrf_gpio_cfg_output(FLASH_PIN_SCK);
nrf_gpio_cfg_output(FLASH_PIN_MOSI);
nrf_gpio_pin_set(FLASH_PIN_CSN);
nrf_gpio_pin_clear(FLASH_PIN_SCK);
nrf_gpio_pin_clear(FLASH_PIN_MOSI);
nrf_gpio_pin_clear(FLASH_PIN_CSN);
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);
nrf_gpio_pin_clear(FLASH_PIN_SCK);
}
nrf_gpio_pin_set(FLASH_PIN_CSN);
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.
5. System OFF Checklist
Before calling sys_poweroff(), all consumers must be powered down:
Expected result: ~44.7 µA (hardware floor of the XIAO BLE Sense)
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 |
7. Flash Workflow (XIAO BLE Sense)
make 099-sysoff-serial-flash BOARD=xiao_ble_sense
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
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 |
