Introduction to Low Power Mode in Bluetooth® Low Energy Scenarios

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This section introduces clock source selection in low power modes for Bluetooth Low Energy (Bluetooth LE), along with common related issues.

Clock Source Selection in Low Power Mode

According to the Bluetooth specification, the sleep clock accuracy must be within 500 PPM. Make sure the clock source selected for Bluetooth LE low power mode meets this requirement. Otherwise, Bluetooth LE may not perform normally and can cause a series of problems, such as ACL connection establishment failure or ACL connection timeout.

Selecting Main XTAL

To select the main XTAL as the Bluetooth LE internal clock source, configure the following option:

CONFIG_BTDM_CTRL_LOW_POWER_CLOCK = Main crystal (CONFIG_BTDM_CTRL_LPCLK_SEL_MAIN_XTAL)

When this is selected, the main XTAL remains powered on during light-sleep, resulting in higher current consumption. Please refer to Power Save README for the typical current consumption in light-sleep using XTAL versus a 32 kHz external crystal.

Selecting 32 kHz External Crystal

To use a 32 kHz external crystal as the Bluetooth LE internal clock source, configure the following options:

Configuration Path 1:

CONFIG_BTDM_CTRL_LOW_POWER_CLOCK = External 32 kHz crystal/oscillator (CONFIG_BTDM_CTRL_LPCLK_SEL_EXT_32K_XTAL)

Configuration Path 2:

CONFIG_RTC_CLK_SRC = External 32 kHz crystal (CONFIG_RTC_CLK_SRC_EXT_CRYS)

Note: Even if 32 kHz is selected in menuconfig, the system will fall back to the main XTAL if the external crystal is not detected during Bluetooth LE initialization. This may lead to unexpected current consumption in light-sleep mode.

Selecting 136 kHz RC Oscillator

Note: ESP32 does not support using 136 kHz RC oscillator as the Bluetooth LE clock source.

How to Check the Current Clock Source Used by Bluetooth LE

You can check the current Bluetooth LE clock source from the initialization logs:

Bluetooth LE Initialization Logs and Clock Sources

Log Message

Clock Source

Using main XTAL as clock source

Main XTAL

Using 136 kHz RC as clock source

Internal 136 kHz RC oscillator

Using external 32.768 kHz crystal as clock source

External 32 kHz crystal

Using external 32.768 kHz oscillator at 32K_XP pin as clock source

External 32 kHz oscillator at 32K_XP pin

FAQ

1. Bluetooth LE ACL Connection Fails or Disconnects in Low Power Mode

As explained in the clock source selection section above, when ACL connections fail to establish or unexpectedly disconnect in low power mode, first verify whether the current clock source meets Bluetooth LE accuracy requirements.

2. Measured light-sleep Current Higher Than Expected

As described in the clock source selection section above, if the main XTAL is used as the clock source, it remains powered on during light-sleep, resulting in higher current consumption than other clock sources. The average current may vary depending on the specific application, Bluetooth LE configuration, and the duration spent in light-sleep. Some applications may have higher average current because Bluetooth LE is active for a larger proportion of the time transmitting and receiving.

3. Unable to Enter light-sleep Mode

If Auto light-sleep is enabled but the device fails to enter light-sleep, it's usually due to insufficient IDLE time, which prevents the automatic entry conditions from being met. This can be caused by excessive logging or Bluetooth LE configurations that reduce IDLE time, such as continuous scanning.

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