Introduction to Low Power Mode for Systemic Power Management

[中文]

The ESP32-S2 supports various low power modes. From a systemic perspective on power management, the typical modes include DFS, Light-sleep mode, and Deep-sleep mode. These modes reduce power consumption by lowering clock frequencies (DFS) or entering sleep states without affecting system functionality. During sleep, unnecessary power domains are shut down, or clock gating is applied to peripherals not in use. Sleep modes are further classified into Light-sleep mode and Deep-sleep mode based on whether powering down domains would disrupt program execution context.

Furthermore, for common use cases of the ESP32-S2 such as Wi-Fi/Bluetooth operation, ESP-IDF segment the modes above and optimize them specifically, which will be introduced in subsequent sections.

This section will first introduce low power modes from a systemic perspective, without considering specific use cases.

DFS

Dynamic Frequency Scaling (DFS) is a fundamental feature of the power management mechanism integrated into ESP-IDF. DFS adjusts the Advanced Peripheral Bus (APB) frequency and CPU frequency based on the application's holding of power locks. When holding a high-performance lock, it utilizes high frequency, while in idle states without holding power locks, it switches to low frequency to reduce power consumption, thereby minimizing the power consumption of running applications as much as possible.

The frequency adjustment mechanism of DFS operates based on the maximum frequency demand dictated by held power locks. Additionally, the values of CONFIG_FREERTOS_HZ also influence the frequency adjustments of DFS. Higher values lead to a higher frequency of task scheduling, then the system can also more quickly re-adjust the clock frequencies according to the system requirements. For further details regarding the frequency adjustment mechanism, please refer to Power Management.

The following graph illustrates the ideal current situation during the operation of the DFS mechanism.

          Hold CPU and APB MAX lock
                    │
                    │    Release CPU MAX lock
           ▲        │      /
  Current  │        ▼     /
           │   ──────────┐       Release APB MAX lock
           │             │           /
           │             │          /
           │             └─────────┐
           │             ▲         │
           │             │         │
           │   m-th tick │         └───────────
           │                       ▲
           │                       │
           │             n-th tick │
           └──────────────────────────────────────►
                                              Time
Ideal DFS Mechanism Frequency Adjustment Current Graph

DFS is suitable for scenarios where the CPU must remain active but low power consumption is required. Therefore, DFS is often activated with other low power modes, as will be detailed in the following sections.

Light-sleep Mode

Light-sleep mode is a low power mode preset in the ESP32-S2. Users can switch to Light-sleep mode by calling esp_light_sleep_start() interface. Upon entering sleep, the chip will shut down unnecessary power domains and apply clock gating to modules not in use, based on the current operational states of peripherals. The ESP32-S2 supports various wake-up sources. Please refer to Sleep Modes for more information. When the chip wakes up from Light-sleep mode, the CPU continues running from the context it was in before entering sleep, and the operational states of peripherals remain unaffected. To effectively reduce chip power consumption under Light-sleep mode, it is highly recommended that users utilize Auto Light-sleep mode described below.

Auto Light-sleep mode is a low power mode provided by ESP-IDF Power Management component that leverages FreeRTOS's Tickless IDLE feature. When the application releases all power locks and all FreeRTOS tasks are in a blocked or suspended state, the system automatically calculates the next time point when an event will wake the operating system. If this calculated time point exceeds a set duration (CONFIG_FREERTOS_IDLE_TIME_BEFORE_SLEEP), the esp_pm component automatically configures the timer wake-up source and enters light sleep to reduce power consumption. To enable this mode, users need to set the light_sleep_enable field to true in esp_pm_config_t when configuring DFS. For more details, please refer to DFS Configuration.

                              ┌────────┐
                              │        │
                              │  DFS   │
                              │        │
                              └───┬────┘
                                  │
                                  ▼
┌──────────┐   when idle      ┌──────────┐ exceed set time ┌──────────┐
│          │  ─────────────►  │          │  ────────────►  │          │
│          │                  │          │                 │   auto   │
│  active  │                  │   IDLE   │                 │  light   │
│          │                  │          │                 │   sleep  │
│          │  ◄─────────────  │          │                 │          │
└──────────┘     not idle     └──────────┘                 └──────┬───┘
  ▲                                                               │
  │                      configure wake-up source                 │
  └───────────────────────────────────────────────────────────────┘

                    Auto Light-sleep Mode Workflow

Based on the workflow of Auto Light-sleep mode, its ideal current graph can be obtained, with key nodes marked on the chart.

        ▲  DFS frequency reduced
Current |        │    exceed set idle time
        |        ▼     / light sleep
        |     ┌──┐    /      ┌──┐
        |     │  └──┐        │  └──┐
        |     |     |        |     |
        |     │     │        │     │
        |  ───┘     └────────┘     └────
        │                     \
        │              wake-up source wakes up
        └───────────────────────────────►
                                     Time
  Ideal Current Graph of Auto Light-sleep Mode

Note

To better illustrate the main changes of Auto Light-sleep mode, the DFS frequency reduction process is omitted from the graph above.
Auto Light-sleep mode is suitable for scenarios where real-time response to external demands is not required.
Auto Light-sleep mode operates based on timer wake-up sources. Therefore, users should not manually configure timer wake-up sources in their application.

Deep-sleep Mode

Deep-sleep mode is designed to achieve better power performance by retaining only RTC/LP memory and peripherals during sleep, while all other modules are shut down. Similar to Light-sleep mode, Deep-sleep mode is entered through API calls and requires configuration of wake-up sources for awakening. Users can switch to Deep-sleep mode by calling esp_deep_sleep_start() interface.

Deep-sleep mode requires the configuration of wake-up sources. The ESP32-S2 supports multiple wake-up sources. For a complete list of wake-up sources, please refer to Sleep Modes. These wake-up sources can also be combined so that any wake-up source can trigger the awakening. If no wake-up source is configured when entering deep sleep, the chip will remain in sleep state until an external reset occurs. Unlike Light-sleep mode, Deep-sleep mode upon awakening will lose the CPU's running context before, so the bootloader needs to be run again to enter the user program.

The workflow of Deep-sleep mode is shown as below:

┌───────┐  call API  ┌───────┐
│       ├───────────►│ deep  │
│active │            │ sleep │
│       │            │       │
└───────┘            └───┬───┘
    ▲                    │
    └────────────────────┘
    wake-up source wakes up
  Deep-sleep Mode Workflow

The primary application scenario of Deep-sleep mode determines that the system will awaken only after a long period and will return to deep sleep state after completing its task. The ideal current graph is as follows.

        ▲
Current |
        |               call API
        |                  │
        |                  ▼
        |               ┌────┐
        |               │    │
        | wake-up source|    |
        |  wakes up  \  |    |
        |             \ │    │
        │     ──────────┘    └────────────────
        │
        └──────────────────────────────────────────►
                                               Time
       Ideal Current Graph of Deep-sleep Mode

Deep-sleep mode can be utilized in low power sensor applications or situations where data transmission is not required for most of the time, commonly referred to as standby mode.

Devices can wake up periodically from deep sleep to measure and upload data, and then return to deep sleep. Alternatively, it can store multiple data sets in RTC memory and transmit them all at once upon the next wake-up. This feature can be implemented using the deep-sleep-stub functionality. For details, please refer to Deep-sleep Wake Stubs.

Low Power Mode Configuration on Pure System

After introducing low power modes from a systemic perspective, this section will present common configuration options, recommended configuration options for each mode, and configurations steps.

Common Configuration Options

Note

The configuration options below are briefly introduced. For more detailed information, please click the link behind each option.

DFS Configuration

DFS offers the following configurable options:

max_freq_mhz
This parameter denotes the maximum CPU frequency (MHz), i.e., the frequency at which the CPU operates at its highest performance level. It is typically set to the maximum value specified by the chip parameters.
min_freq_mhz
This parameter denotes the minimum CPU frequency (MHz), i.e., the CPU's operating frequency when the system is in an idle state. This field can be set to the crystal oscillator (XTAL) frequency value or the XTAL frequency value divided by an integer.
light_sleep_enable
Enabling this option allows the system to automatically enter the light sleep during idle periods, i.e., enabling Auto Light-sleep mode, as detailed earlier.

Specific configuration steps are as follows:

Enable CONFIG_PM_ENABLE
Configure max_freq_mhz and min_freq_mhz as follows:

esp_pm_config_t pm_config = {
        .max_freq_mhz = CONFIG_EXAMPLE_MAX_CPU_FREQ_MHZ,
        .min_freq_mhz = CONFIG_EXAMPLE_MIN_CPU_FREQ_MHZ,
        .light_sleep_enable = false
};
ESP_ERROR_CHECK(esp_pm_configure(&pm_config));

Recommended Configuration

Configuration Name	Configuration Status
Enable power management component (CONFIG_PM_ENABLE)	ON
RTOS Tick rate (Hz) (CONFIG_FREERTOS_HZ)	1000
`max_freq_mhz`	160
`min_freq_mhz`	40
`light_sleep_enable`	false

Note

Configurations not mentioned in the above table are set to default.

Light-sleep Mode Configuration

This section introduces the recommended configuration and configuration steps for Auto Light-sleep mode.

Note

The configuration options below are briefly introduced. For more detailed information, please click the link behind each option.

Minimum IDLE Tick count before entering sleep state (CONFIG_FREERTOS_IDLE_TIME_BEFORE_SLEEP)
Put light sleep related codes in IRAM (CONFIG_PM_SLP_IRAM_OPT)
Put RTOS IDLE related codes in IRAM (CONFIG_PM_RTOS_IDLE_OPT)
RTC slow clock source (CONFIG_RTC_CLK_SRC)

Clock Source

Timer Accuracy

Frequency Offset

RTC_CLK_SRC_INT_RC

High

Large

RTC_CLK_SRC_EXT_CRYS

Low

Small
Disable all GPIO when chip at sleep (CONFIG_PM_SLP_DISABLE_GPIO)

Power down flash in light sleep (CONFIG_ESP_SLEEP_POWER_DOWN_FLASH)

Due to the shared power pins between flash and PSRAM, cutting power to PSRAM would result in data loss. Therefore, to ensure light sleep does not disrupt program execution, enabling this option requires that the system does not utilize PSRAM.

Configuration Steps:

Configure wake-up sources (refer to Sleep Modes for details)
Enable CONFIG_PM_ENABLE
Enable CONFIG_FREERTOS_USE_TICKLESS_IDLE
Configure DFS parameters
light_sleep_enable = true, detailed as follows:

esp_pm_config_t pm_config = {
  .max_freq_mhz = CONFIG_EXAMPLE_MAX_CPU_FREQ_MHZ,
  .min_freq_mhz = CONFIG_EXAMPLE_MIN_CPU_FREQ_MHZ,
  #if CONFIG_FREERTOS_USE_TICKLESS_IDLE
  .light_sleep_enable = true
  #endif
};
ESP_ERROR_CHECK(esp_pm_configure(&pm_config));

Additional relevant parameters for configuration introduction

Recommended Configuration

Configuration Name	Configuration Status
Enable power management component (CONFIG_PM_ENABLE)	ON
Enable RTOS Tickless IDLE mode (CONFIG_FREERTOS_USE_TICKLESS_IDLE)	ON
RTOS Tick rate (Hz) (CONFIG_FREERTOS_HZ)	1000
Minimum IDLE Tick count before entering sleep mode (CONFIG_FREERTOS_IDLE_TIME_BEFORE_SLEEP)	3
Put light sleep related codes in IRAM (CONFIG_PM_SLP_IRAM_OPT)	OFF
Put RTOS IDLE related codes in IRAM (CONFIG_PM_RTOS_IDLE_OPT)	OFF
RTC slow clock source (CONFIG_RTC_CLK_SRC)	Internal 150 kHz OSC
Disable all GPIO when chip at sleep (CONFIG_PM_SLP_DISABLE_GPIO)	ON
`max_freq_mhz`	160
`min_freq_mhz`	40
`light_sleep_enable`	true

Note

Configurations not mentioned in the above table are set to default.

Deep-sleep Mode Configuration

For Deep-sleep mode, other configurations are of minimal significance except wake-up source-related configurations.

Configuration Steps:

Configure wake-up sources (refer to Sleep Modes for details)
Call the API, as follows

/* Enter deep sleep */
esp_deep_sleep_start();

Users can keep specific modules powered on during sleep using the following configuration options:

Power up External 40 MHz XTAL
In some special applications, certain modules require high clock accuracy and stability during sleep (e.g., BT). In such cases, it is recommended to enable the External 40 MHz XTAL during sleep. Code to enable and disable, as follows:
ESP_ERROR_CHECK(esp_sleep_pd_config(ESP_PD_DOMAIN_XTAL, ESP_PD_OPTION_ON)); ESP_ERROR_CHECK(esp_sleep_pd_config(ESP_PD_DOMAIN_XTAL, ESP_PD_OPTION_OFF));
Power up Internal 8 MHz OSC
In some special applications, certain modules (e.g., LEDC) use the Internal 8 MHz OSC as a clock source and need to function normally during light sleep. In such cases, it is recommended to enable the Internal 8 MHz OSC during sleep. Code to enable and disable, as follows:
ESP_ERROR_CHECK(esp_sleep_pd_config(ESP_PD_DOMAIN_RTC8M, ESP_PD_OPTION_ON)); ESP_ERROR_CHECK(esp_sleep_pd_config(ESP_PD_DOMAIN_RTC8M, ESP_PD_OPTION_OFF));

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Clock Source	Timer Accuracy	Frequency Offset
RTC_CLK_SRC_INT_RC	High	Large
RTC_CLK_SRC_EXT_CRYS	Low	Small