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Compare Boot Time and Energy Consumption for sending Messages via ESPNow and Wifi

We will compare the boot time and energy consumption of ESP32 devices to:

  • Wake from deepsleep,
  • Boot to Micropython,
  • Execute frozen module to:
    • Send a status message via ESPNow or Wifi,
    • Return to deepsleep.

These measurements use the optimised (fast boot) process described in Optimising Micropython Boot Time.

Commit: c34ca002 Add frozen modules to send status over espnow/wifi/mqtt.

Summary of Results

(Energy (uWh) = 1000 * Charge (mC) * 5V / (60 * 60))

Test Time (ms) Charge (mC) Energy (microWh)
Fast Boot to _boot.py only 70 3.32 4.6
Send ESPNow Succeed 119 7.68 10.7
Send ESPNow Failed 145 15.4 21.4
Connect to wifi (DHCP) and send HTTP Post 2928 341.3 474.0
Connect to wifi (DHCP) 1626 204.3 283.8
Connect to wifi (Static IP) 734 100.2 139.2
Connect to wifi (Static IP) and send MQTT message 774 108.6 150.9

Send status via ESNow from _boot.py (commit fc62686)

NOTES:

  • This commit has nvs_flash_init() enabled at boot time as this is required for esp_wifi_init() (additional 7ms of bootup time).
  • This app is run as a module frozen into the Micropython image
    • Add ~350ms (23.6mC, 32.8uWh) to mount filesystem from flash storage and execute app.

_preboot.py:

import machine

def send_state(broker):
    import network
    from _espnow import ESPNow

    enow = ESPNow()
    enow.active(True)
    enow.add_peer(broker)
    sta = network.WLAN(network.STA_IF)
    sta.active(True)
    enow.send(broker, b"wake_up", True)
    enow.active(False)
    sta.active(False)

if machine.reset_cause() == machine.DEEPSLEEP_RESET:
    send_state(b"\xf4\x12\xfaA\xf7T")
    machine.deepsleep(1000)

NOTE: The ESP32 board has a deepsleep current of 15mA.
ESP32 Boot, Send ESPNow message: Time to boot and return to deepsleep is 118.8ms (7.68mC). Sending the message generates a current spike of ~550mA over 0.6ms (1 byte) to 2.5ms (255 bytes). _
If the peer is not found, the transmission is retried and more energy is consumed: 144.9ms (15.43mC) (twice the energy consumption). Note: Retransmission does NOT occur if sending to the broadcast address. _
Zoom in on retransmit attempts _

Send status over wifi from _boot.py (commit 44a1341)

_preboot_wifi.py:

import machine

def send_state(broker):
    import network
    import urequests

    sta = network.WLAN(network.STA_IF)
    sta.active(True)
    sta.connect("ssid", "password")
    while not sta.isconnected():
        pass
    try:
        r = urequests.post("http://XXX.XXX.XXX.XXX:5000/status", data="hello")
    except OSError:
        pass
    sta.disconnect()
    sta.active(False)

if machine.reset_cause() == machine.DEEPSLEEP_RESET:
    send_state()
    machine.deepsleep(1000)

NOTES:

  • Boot times showed considerable variability with some transmissions taking up to 4.9s (490mC).
  • Wifi connection and dhcp response times may vary by Access Point.
  • You can see the WIFI_PS_MIN_MODEM power saving mode at work, where the wifi radio is turned off periodically while connected to the Access Point.
ESP32 Boot, connect to wifi and DHCP, POST message: Time to boot and return to deepsleep is 2928ms (341.3mC) . _
ESP32 Boot, connect to Wifi and DHCP: Time to connect to wifi and then deepsleep (ie. don't send message) is 1626ms (204.3mC). _
ESP32 Wifi connect - static IP: Time to boot and return to deepsleep is 734.3ms (100.2mC) _
ESP32 Wifi connect - MQTT - Static IP: Time to boot and post MQTT message is 774ms (108.6mC) _

_preboot_mqtt.py:

import machine

def send_state(broker):
    import network
    import urequests
    from umqtt.simple import MQTTClient, MQTTException

    sta = network.WLAN(network.STA_IF)
    sta.active(True)
    sta.connect("ssid", "password")
    while not sta.isconnected():
        pass
    try:
        mq = MQTTClient(
            "esp32_test", "192.168.10.10",
            user="mqtt_user", password="mqtt_password")
        mq.connect()
        mq.publish("glenn_test/test", "Sensor")
        mq.disconnect()
    except MQTTException as err:
        print(err)
    sta.disconnect()
    sta.active(False)

if machine.reset_cause() == machine.DEEPSLEEP_RESET:
    send_state()
    machine.deepsleep(1000)