ESP32 power saving: modem and light sleep – 2

ESP32 can activate light sleep and deep sleep modes with a specific command, but there is another modem sleep mode; this is not a well-defined suspend like the others. It may be necessary to perform more than one operation to implement it.

So remember that modem sleep mode is virtual, not executed with a specific command, and to understand better, it is helpful to read the previous chapter.

The light sleep mode is, on the other hand, well defined, and there is a specific command to activate it. With this mode, the digital devices, most of the RAM, and the CPUs have a reduced clock frequency, and also the voltage is reduced. Upon exiting light sleep mode, peripherals and CPUs resume functioning, preserving their internal state.

In this table, taken from the esp32 datasheet, there is information on how to group sleep modes.

How to measure ampere

Now we will check how much power consumes the ESP32 in many situations and configurations. Here is the connection schema we used.

Power the esp32 to the 5v pin and GND pin with an external power supply and disconnect the USB. To log, we use the Serial2 port, but if you want to use the Serial port, you must only move the FTDI converter to the TX pin instead TX2 pin. You can’t use USB because it powers the device, and the multimeter gets the wrong amperage.

Modem sleep

If WiFi connection needs to be maintained, enable WiFi modem sleep, and enable automatic light sleep feature. This will allow the system to wake up from sleep automatically when required by WiFi driver, thereby maintaining connection to the AP.

From Espressif documentation

The modem sleep mode is quite ambiguous. Some people/blogs claim it’s WiFi sleep mode (and Espressif confirms it), others that it’s when all radios (WiFi and Bluetooth) are disabled.

Real

So if we are going to read the Espressif documentation, the real modem sleep It’s the regular operation of WiFi (check the previous chapter), here is a code example:

/*
 *  ESP32
 *  REAL Modem Sleep and wake up
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org/category/tutorial/esp32-tutorial/esp32-practical-power-saving/
 *
 */

#include <WiFi.h>
#include <BluetoothSerial.h>
#include "driver/adc.h"
#include <esp_bt.h>

#define STA_SSID "<YOUR-SSID>"
#define STA_PASS "<YOUR-PASSWD>"

BluetoothSerial SerialBT;

void setModemSleep();
void wakeModemSleep();

void setup() {
	Serial2.begin(115200);

	while(!Serial2){delay(500);}

  	SerialBT.begin("ESP32test"); //Bluetooth device name
	SerialBT.println("START BT");

	Serial2.println("START WIFI");
	WiFi.begin(STA_SSID, STA_PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial2.print(".");
    }

    Serial2.println("");
    Serial2.println("WiFi connected");
    Serial2.println("IP address: ");
    Serial2.println(WiFi.localIP());

    setModemSleep();
    Serial2.println("MODEM SLEEP ENABLED FOR 5secs");
}

//void loop() {}
unsigned long startLoop = millis();
bool started = false;
void loop() {
	if (!started && startLoop+5000<millis()){
		// Not use delay It has the own policy
		wakeModemSleep();
		Serial2.println("MODEM SLEEP DISABLED");
		started = true;
	}
}


void setModemSleep() {
	WiFi.setSleep(true);
	if (!setCpuFrequencyMhz(40)){
		Serial2.println("Not valid frequency!");
	}
	// Use this if 40Mhz is not supported
	// setCpuFrequencyMhz(80);
}

void wakeModemSleep() {
	setCpuFrequencyMhz(240);
}

And result in power consumption of 40mA ~ 68mA, the minimum when automatic sleep has activated the max when receiving the beacon message.

Fake

Here is the most common (fake) interpretation of the meaning of the term modem sleep.

If you have already read the first chapter, you know that you can disable WiFi with these commands

void disableWiFi(){
        adc_power_off();
	WiFi.disconnect(true);  // Disconnect from the network
	WiFi.mode(WIFI_OFF);    // Switch WiFi off
}

To reduce consumption, you can also disable other units in the Radio group like the BlueTooth with this command

void disableBluetooth(){
	btStop();
}

But as we have seen, the only advantage of turning off WiFi is that it is susceptible to beacon messages from the network because the default is a WiFi sleep mode which puts the WiFi into automatic sleep mode for inactivity and is equivalent to disabling the modem.

Then, you must pay attention to the datasheet table that you can obtain better modem sleep by modifying the CPU frequencies (view the previous chapter), reducing the MHz of the CPU

	setCpuFrequencyMhz(40);

So now we know what we must do to go on Modem Sleep:

• Disable WiFi;
• Disable Bluetooth;
• Reduce CPU frequency.

The resulting code is

/*
 *  ESP32
 *  Modem Sleep and wake up
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org/category/tutorial/esp32-tutorial/esp32-practical-power-saving/
 *
 */

#include <WiFi.h>
#include <BluetoothSerial.h>
#include "driver/adc.h"
#include <esp_bt.h>

#define STA_SSID "<YOUR-SSID>"
#define STA_PASS "<YOUR-PASSWD>"

BluetoothSerial SerialBT;

void setModemSleep();
void wakeModemSleep();

void setup() {
	Serial2.begin(115200);

	while(!Serial2){delay(500);}

  	SerialBT.begin("ESP32test"); //Bluetooth device name
	SerialBT.println("START BT");

	Serial2.println("START WIFI");
	WiFi.begin(STA_SSID, STA_PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial2.print(".");
    }

    Serial2.println("");
    Serial2.println("WiFi connected");
    Serial2.println("IP address: ");
    Serial2.println(WiFi.localIP());

    setModemSleep();
    Serial2.println("MODEM SLEEP ENABLED FOR 5secs");
}

//void loop() {}
unsigned long startLoop = millis();
bool started = false;
void loop() {
	if (!started && startLoop+5000<millis()){
		// Not use delay It has the own policy
		wakeModemSleep();
		Serial2.println("MODEM SLEEP DISABLED");
		started = true;
	}
}

void disableWiFi(){
    adc_power_off();
	WiFi.disconnect(true);  // Disconnect from the network
	WiFi.mode(WIFI_OFF);    // Switch WiFi off
	Serial2.println("");
	Serial2.println("WiFi disconnected!");
}
void disableBluetooth(){
	// Quite unusefully, no relevable power consumption
	btStop();
	Serial2.println("");
	Serial2.println("Bluetooth stop!");
}

void setModemSleep() {
	disableWiFi();
	disableBluetooth();
	setCpuFrequencyMhz(40);
	// Use this if 40Mhz is not supported
	// setCpuFrequencyMhz(80);
}

void enableWiFi(){
	adc_power_on();
	delay(200);

	WiFi.disconnect(false);  // Reconnect the network
	WiFi.mode(WIFI_STA);    // Switch WiFi off

	delay(200);

	Serial2.println("START WIFI");
	WiFi.begin(STA_SSID, STA_PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial2.print(".");
    }

    Serial2.println("");
    Serial2.println("WiFi connected");
    Serial2.println("IP address: ");
    Serial2.println(WiFi.localIP());
}

void wakeModemSleep() {
	setCpuFrequencyMhz(240);
	enableWiFi();
}

With Modem Sleep, you can obtain a considerable power reduction, and you can reach 20mA and lesser with a 40Mhz (check the previous chapter).

WeMos LOLIN32 and TTGO testing

I want to use other microcontrollers for the next test because manufacturers often create these boards without thinking about energy saving. Therefore we must always check if these devices are suitable for production or are just testing.

And we need to pay attention to microcontrollers with built-in battery management because they can be satisfying when battery powered. Manufacturers manage functional units to get better battery performance, but let’s check the difference with the multimeter.

Light sleep

Light sleep is different from modem sleep because you are about to pause the CPU, so you have to manage that state with a particular command, and you must first give a system wake-up mode.

Light sleep can fail if there are some active connections (WiFi or Bluetooth), so you probably must disable WiFi and Bluetooth to ensure they do not fail.

In detail, in light sleep mode digital peripherals, most of the RAM and CPUs have a reduced clock frequency, and also the voltage is reduced. Upon exiting soft sleep mode, peripherals and CPUs resume functioning, preserving their internal state.

If you do not select a peripheral as the wake-up mode

Please pay attention to these two sketches; the first seems similar to the second, but there is a big difference in power consumption, and now we will explain better.

DOIT esp32 DEV KIT v1

Now we are going to put in light sleep a DOIT esp32 DEV KIT v1 with this sketch:

/*
 *  ESP32
 *  Light Sleep and wake up
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org/category/tutorial/esp32-tutorial/esp32-practical-power-saving/
 *
 */

#include <WiFi.h>
#include <BluetoothSerial.h>
#include "driver/adc.h"
#include <esp_bt.h>
#include <esp_wifi.h>
#include <esp_sleep.h>
#define STA_SSID "<YOUR-SSID>"
#define STA_PASS "<YOUR-PASSWD>"

BluetoothSerial SerialBT;

int variable = 0;

void setup() {
	Serial2.begin(115200);

	while(!Serial2){delay(500);}

  	SerialBT.begin("ESP32test"); //Bluetooth device name
	SerialBT.println("START BT");

	Serial2.println("START WIFI");
	WiFi.begin(STA_SSID, STA_PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial2.print(".");
    }

    Serial2.println("");
    Serial2.println("WiFi connected");
    Serial2.println("IP address: ");
    Serial2.println(WiFi.localIP());

    delay(1000);

    variable += 10;

    Serial2.println();
    Serial2.println("LIGHT SLEEP ENABLED FOR 5secs");
    delay(100);

    esp_sleep_enable_timer_wakeup(5 * 1000 * 1000);
    esp_light_sleep_start();

    Serial2.println();
    Serial2.println("LIGHT SLEEP WAKE UP");

    Serial2.print("Variable = ");
    Serial2.println(variable);
}

void loop() {

}

void disableWiFi(){
    adc_power_off();
	WiFi.disconnect(true);  // Disconnect from the network
	WiFi.mode(WIFI_OFF);    // Switch WiFi off

	Serial2.println("");
	Serial2.println("WiFi disconnected!");
}
void enableWiFi(){
	adc_power_on();
	WiFi.disconnect(false);  // Reconnect the network
	WiFi.mode(WIFI_STA);    // Switch WiFi off

	Serial2.println("START WIFI");
	WiFi.begin(STA_SSID, STA_PASS);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial2.print(".");
    }

    Serial2.println("");
    Serial2.println("WiFi connected");
    Serial2.println("IP address: ");
    Serial2.println(WiFi.localIP());
}
void disableBluetooth(){
	btStop();
	esp_bt_controller_disable();
	delay(1000);
	Serial2.println("BT STOP");
}

DOIT DEV KIT v1

But the result is not so good; this device in light sleep consumes 4.89mA, very far from the 0.8 mA declared from Espressif.

As you can see, the variable’s value is ten that was set before entering the light sleep. These are the most critical features of light sleep, the program is suspended, and the state is restored.

At first, I was depressed, but then I reflected on the fact that that figure was related to the esp32 chip, and that’s it.
But in a dev board, there are a lot of other components that can change the result. Think of LEDs, voltage regulators, etc.

So I try to do the same test with two famous boards vendors that have devices with battery management and other features, not only for dev.

TTGO T8

First, I want to try a TTGO, one of the most famous productor of this type of device, with many variants; I use one of the better of Its products, the TTGO T8.

The power source at 5v from the specified pin gives a lousy result of 8.24mA. Still, the board, when powered only with the battery, excludes some functional units like a led voltage regulator, etc., so the same test with battery power reach 3.73mA, relatively better than the first result and much better than the DOIT DEV KIT v1.

WeMos LOLIN32

Then the WeMos LOLIN32, I love WeMos as a manufacturer; I think It has a good set of devices and is very versatile with different sizes.

The power source at 5v from the specified pin gives a relatively better result, 2.60mA. Still, when powered only with the battery, the board reaches 1.52mA, which is a satisfactory result.

Result

The data is obvious, the DOIT DEV KIT v1 is the loser, and the LOLIN32 is the winner; TTGO has the highest price, I don’t think it’s satisfactory.

RTC and other wake up source

But we didn’t use RTC as a wake-up source, and if you pay attention to the schematic, RTC has a different position in the functional modules.

But other wake-up sources (such as the external one) are located in different functional blocks. So what happens if we use a wake source like RTC block?

We will use the previous sketch, but we will add an external wake-up source.

    esp_sleep_enable_timer_wakeup(5 * 1000 * 1000);
    esp_sleep_enable_ext0_wakeup(GPIO_NUM_33,1);

    esp_light_sleep_start();

and here is the result

RTC IO module contains logic to trigger wakeup when one of RTC GPIOs is set to a predefined logic level. RTC IO is part of RTC peripherals power domain, so RTC peripherals will be kept powered on during deep sleep if this wakeup source is requested.

From Espressif documentation

Please pay attention to these words as they will explain the difference between hibernation and deep sleep.

Thanks

In this article, we learn that different devices have different results, so when you buy a device, remember to check the quality.

1. ESP32: pinout, specs and Arduino IDE configuration
2. ESP32: integrated SPIFFS Filesystem
3. ESP32: manage multiple Serial and logging
4. ESP32 practical power saving
   1. ESP32 practical power saving: manage WiFi and CPU
   2. ESP32 practical power saving: modem and light sleep
   3. ESP32 practical power saving: deep sleep and hibernation
   4. ESP32 practical power saving: preserve data, timer and touch wake up
   5. ESP32 practical power saving: external and ULP wake up
   6. ESP32 practical power saving: UART and GPIO wake up
5. ESP32: integrated LittleFS FileSystem
6. ESP32: integrated FFat (Fat/exFAT) FileSystem
7. ESP32-wroom-32
   1. ESP32-wroom-32: flash, pinout, specs and IDE configuration
8. ESP32-CAM
   1. ESP32-CAM: pinout, specs and Arduino IDE configuration
   2. ESP32-CAM: upgrade CamerWebServer with flash features
9. ESP32: use ethernet w5500 with plain (HTTP) and SSL (HTTPS)
10. ESP32: use ethernet enc28j60 with plain (HTTP) and SSL (HTTPS)
11. How to use SD card with esp32
12. esp32 and esp8266: FAT filesystem on external SPI flash memory

13. Firmware and OTA update management
    1. Firmware management
       1. ESP32: flash compiled firmware (.bin)
       2. ESP32: flash compiled firmware and filesystem (.bin) with GUI tools
    2. OTA update with Arduino IDE
       1. ESP32 OTA update with Arduino IDE: filesystem, firmware, and password
    3. OTA update with Web Browser
       1. ESP32 OTA update with Web Browser: firmware, filesystem, and authentication
       2. ESP32 OTA update with Web Browser: upload in HTTPS (SSL/TLS) with self-signed certificate
       3. ESP32 OTA update with Web Browser: custom web interface
    4. Self OTA uptate from HTTP server
       1. ESP32 self OTA update firmware from the server
       2. ESP32 self OTA update firmware from the server with version check
       3. ESP32 self-OTA update in HTTPS (SSL/TLS) with trusted self-signed certificate
    5. Non-standard Firmware update
       1. ESP32 firmware and filesystem update from SD card
       2. ESP32 firmware and filesystem update with FTP client

14. Integrating LAN8720 with ESP32 for Ethernet Connectivity with plain (HTTP) and SSL (HTTPS)
15. Connecting the EByte E70 to ESP32 c3/s3 devices and a simple sketch example
16. ESP32-C3: pinout, specs and Arduino IDE configuration
17. Integrating W5500 with ESP32 Using Core 3: Native Ethernet Protocol Support with SSL and Other Features
18. Integrating LAN8720 with ESP32 Using Core 3: Native Ethernet Protocol Support with SSL and Other Features