ESP32 practical power saving: manage WiFi and CPU – 1

The ESP32 can activate the energy-saving modes of light sleep and deep sleep, but in many cases, we cannot use such a solution. However, it is still possible to apply some options to the functional units for reducing energy consumption.

You can find various type of ESP32 to ESP32 Dev Kit v1 - TTGO T-Display 1.14 ESP32 - NodeMCU V3 V2 ESP8266 Lolin32 - NodeMCU ESP-32S - WeMos Lolin32 - WeMos Lolin32 mini - ESP32-CAM programmer - ESP32-CAM bundle - ESP32-WROOM-32 - ESP32-S

In light sleep mode, digital peripherals, most RAM, and CPUs are clock-gated, and supply voltage is reduced. Upon exit from light sleep, peripherals and CPUs resume operation, their internal state is preserved.

In deep sleep mode, CPUs, most of the RAM, and all the digital peripherals clocked from the clock of the Advanced Peripheral Bus (APB_CLK) are powered off. The only parts of the chip which can still be powered on are real-time clock (RTC) controller, RTC peripherals, including Ultra-Low Power (ULP) coprocessor, and RTC memories (slow and fast).

In this table, you can see the table from the datasheet of esp32 with information on how sleep mode is grouped.

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.

For the Serial reading, you can refer to this article “ESP32: manage multiple Serial and logging for debugging“.

WiFi management

A system to manage battery endurance is to work on a single microcontroller unit. As you see in the upper schema, there are a lot of functional units that you don’t use or can manage.

Reduce communication with WiFi

Turning on WiFi or Bluetooth consumes amps, but it is when they communicate that they consume a lot of power, as you can see in this table:

So the first thing you can do is reduce the communication time to the minimum possible.

WiFi: enable power saving modes or get best performance

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

You can manage WiFi power saving management, and there are three possibilities that you can set.

• WIFI_PS_NONE: disable modem sleep entirely;
• WIFI_PS_MIN_MODEM: enable Modem-sleep minimum power save mode;
• WIFI_PS_MAX_MODEM: to enable Modem-sleep maximum power save mode.

WIFI_PS_MIN_MODEM is the default parameter; when modem sleep is enabled, received WiFi data can be delayed for as long as the DTIM period.

A delivery traffic indication map (DTIM) is a kind of TIM that informs the clients about the presence of buffered multicast/broadcast data on the access point. It is generated within the periodic beacon at a frequency specified by the DTIM Interval. Beacons are packets sent by an access point to synchronize a wireless network. Normal TIMs that are present in every beacon are for signaling the presence of buffered unicast data. After a DTIM, the access point will send the multicast/broadcast data on the channel following the standard channel access rules.

The problem of the DTIM period is that if you have a lot of traffic in your network, you may receive many beacons messages.

WIFI_PS_MAX_MODEM, when modem sleep is enabled, received WiFi data can be delayed for as long as the listen interval duration. This feature is hidden by the default E32 core because there isn’t a significant gain with this technique.

WIFI_PS_NONE disable modem sleep entirely. This has much higher power consumption but provides minimum latency for receiving WiFi data in real-time. 

With my esp32 DEV KIT v1, I obtain this power-consuming difference.

But now we are going to the more exciting part, managing this kind of power savings.

Take this code as an example.

/*
 *  ESP32
 *  Manage WiFi sleep mode
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org
 *
 * You may copy, alter and reuse this code in any way you like, but please leave
 * reference to www.mischianti.org in your comments if you redistribute this code.
 */

#include <WiFi.h>
#include <BluetoothSerial.h>

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

BluetoothSerial SerialBT;

void setWiFiPowerSavingMode();

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(".");
    }

    setWiFiPowerSavingMode();

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

void loop() {

}

void setWiFiPowerSavingMode(){
    WiFi.setSleep(true);
}

With this code, you are set to default parameter, so you set WIFI_PS_MIN_MODEM; if you remove the line setWiFiPowerSavingMode(); the power consumption remains the same, about 80mA ~ 108mA, the min consumption is when no beacon messages are present (so sleep), the max is when beacon messages arrive.

If you change the function like so

void setWiFiPowerSavingMode(){
    WiFi.setSleep(false);
}

the power consumption changes dramatically and increases to 145mA. With the instruction you have set, WIFI_PS_NONE, there is no latency in your WiFi and increases the packet transfer rate.

As I have already written, the third WIFI_PS_MAX_MODEM mode is “hidden” to enable it. You need to import a new package and use primitive instructions.

#include <esp_wifi.h>
[...]
void setWiFiPowerSavingMode(){
    esp_wifi_set_ps(WIFI_PS_MAX_MODEM);
    //    esp_wifi_set_ps(WIFI_PS_NONE);
    //    esp_wifi_set_ps(WIFI_PS_MIN_MODEM);
}

It seems there isn’t a difference, but the interval of check message becomes less frequently because It checks communications at regular intervals (not when beacon messages arrive). The power consumption remains similar to 80mA ~ 108mA and does not change your battery problems, but it could make a difference in a dynamic network.

Disable WiFi

If you don’t need WiFi, whenever you can stop it with the specified command,

It is also good to disable the ADC; otherwise, you may have problems. The ESP32 integrates two 12-bit SAR (Successive Approximation Register) ADCs supporting a total of 18 measurement channels (analog enabled pins).
The WiFi driver uses ADC2. Therefore the application can only use ADC2 when the WiFi driver has not started.

Here is a simple example.

/*
 *  ESP32
 *  Disable WiFi
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org
 *
 * You may copy, alter and reuse this code in any way you like, but please leave
 * reference to www.mischianti.org in your comments if you redistribute this code.
 */

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

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

BluetoothSerial SerialBT;

void disableWiFi();

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());

    disableWiFi();
    Serial2.println("WiFi disconnected");
}

void loop() {

}

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

As you can see, the power consumption remains similar, about 72mA (only 8 ~ 12 mA), but the real advantage is when a beacon message arrives, there is no fluctuation in power consumption anymore, so when beacon message travel on the network, the benefit become 18 ~ 46 mA.

To restore WiFi, the function is:

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());
}

CPU management

Another critical feature to reduce power consumption is cutting the CPU frequencies.

Exist a simple method to get and set CPU frequencies, getCpuFrequencyMhz() and setCpuFrequencyMhz(80).

The only value admitted (check the table) is 240, 160, and 80, but I tried with lesser value, and It seems. It works correctly (check the video with 40Mhz, 20Mhz, and 10MHz).

Here is a simple example of how to set 80Mhz as a frequency.

/*
 *  ESP32
 *  Manage CPU Frequences
 *  by Mischianti Renzo <https://mischianti.org>
 *
 *  https://mischianti.org
 *
 * You may copy, alter and reuse this code in any way you like, but please leave
 * reference to www.mischianti.org in your comments if you redistribute this code.
 */

#include "Arduino.h"

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

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

	Serial2.print("CPU Freq: ");
	Serial2.println(getCpuFrequencyMhz());

	setCpuFrequencyMhz(80);

	Serial2.print("CPU Freq: ");
	Serial2.println(getCpuFrequencyMhz());
}

void loop()
{

}

I do some tests, as you can see in the video, and I verify that without any management, the consumption is about 66.8mA.

By setting

setCpuFrequencyMhz(160);

is 45.9mA, by setting

setCpuFrequencyMhz(80);

the power become 33.2mA, and with

setCpuFrequencyMhz(40);

è 19.88mA, then I try to set

setCpuFrequencyMhz(20);

the power become 15.43mA with

setCpuFrequencyMhz(10);

and the power becomes 13.19mA.

Thanks

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2. ESP32: integrated SPIFFS Filesystem
3. ESP32: manage multiple Serial and logging
4. ESP32 practical power saving
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   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
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    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
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       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

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