uPesy ESP32 Wroom Low Power DevKit v1.2

ESP32 module

The brain of the board is the ESP32: a powerful microcontroller made by Espressif . A WROOM module is used (as suggested in the board name 🤔). It contains an ESP32 chip and a primary circuit to run it.

You will find all the technical information you need in the manufacturer’s datasheets:

• For the WROOM Module https://www.espressif.com/sites/default/files/documentation/esp32-wroom-32_datasheet_en.pdf

• For ESP32 https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf

USB/UART converter

Since the ESP32 does not understand the USB protocol, a Silicon Labs chip, the CP2102 acts as a translator with the computer, he’s natively supported by Windows, macOS and Linux, so there is no need to install an additional driver, as would be with a CH340G.

Power management

This section of the board is responsible for power management. It contains, among other things, a 3.3V voltage regulator and protection. Its operation will be described in detail in the next section.

Connectivity

The USB-C connector allows powers the board and communicates with the computer. It will enable the user to upload a program and print messages in the console …

The two buttons EN and BOOT of the board allow to control the state of the ESP32:

• EN : This button, sometimes called RESET, is used to restart the ESP32 forcefully.

• BOOT : This button alone has no effect when the ESP32 is already turned on. It acts on its behavior during the boot process.

These two buttons can manually put the ESP32 in FLASH mode, which allows loading a new program on the ESP32. Usually, the ESP32 automatically goes into this mode when the upload is done via USB with the CP2102 converter.

To do this, when the connection stage with the ESP32 reaches the:

Connecting........_____....._____....._____....._____....._____....._____....._____....._____....._____....._____

The following combination is required:

• Press and hold the EN and BOOT buttons simultaneously for 2 seconds.

• Release the EN button while holding down BOOT for 2 seconds.

• Release the BOOT button

  

Voltage tolerance of the pins

• The ESP32 is a microcontroller that works in 3.3V. The logic levels are, therefore, 0 and 3.3V and not 0 and 5V. That is to say that the output voltage of the GPIO pins is 3.3V, and the input voltage must not exceed 3.3V. The GPIO pins are not designed to have logic levels of 5V .

• The voltage measured by the analog-to-digital converter must not exceed 3.3V (and not negative, of course).

Different Power Mode

There are three ways to power the ESP32 Low Power board. Two are optimized for low current consumption:

• Via USB (\(V_{BUS}\) ): The board is connected via its USB-C connector to a computer or a suitable phone charger. A red LED lights up as soon as the power is turned on. This power supply mode is not optimized for the consumption of current.

• Via an external battery (\(V_{BAT}\) ): We connect a Lithium-ion/LiPo battery of 3.7V on the JST2.0 connector. If we combine the USB simultaneously, the board charges the battery. This power supply mode is optimized for the current consumption: the consumption will be <15µA in Deep Sleep.

  

• Via an external power supply (\(V_{IN}\) ): We connect on the pin \(V_{IN}\) of the ESP32 board an external power supply with a voltage between 3.5V and 7V maximum . This power supply mode is optimized for the current consumption: the consumption will be <15µA in Deep Sleep.

Battery type to use

The battery must be compatible with the integrated charger of the uPesy board. The charger is IC an MCP73831 from MicroChip , which allows recharging Li-Ion/Li-Polymer batteries of a single cell of 3.7V (or 4.2V when the battery is fully charged).

They are widely used on mobile devices.

The battery’s capacity will depend on your application and how long you need to recharge the battery. A capacity of 1000mAh or 2000mAh will be more than enough in most cases.

The other important point is to check that the polarity of your battery matches that of the board.

Please take a look at the picture above. To check the polarity of the 3.7V battery, the red wire must be plugged on the “+” side.

Here are examples of batteries compatible with the uPesy board:

• https://www.amazon.com/dp/B08ZCQXFX4

• https://www.amazon.com/dp/B087LTZW61 [JST 1.25 connector]

• https://www.amazon.com/dp/B082152887 [reversed polarity]

The battery charging process

Recharging a battery already connected to the JST connector is done as follows :

• The battery no longer powers the board when a USB cable is connected. It is the USB power supply that will both power the board and charge the battery in addition.

  Note

  It’s better to connect the battery first, then the USB. Sometimes, on the other way, the charger does not start charging.

• The blue LED should light up. This LED will only light up while the battery is being charged.

• As long as the blue LED is on, the battery is charging.

• When this LED is off, the battery is fully charged.

• The USB cable can be disconnected, and the battery will power the ESP32 again.

The maximum charging current is limited to 500 mA.

For an empty 1000mAh battery, the total charge time is around 3h30. 70% of the battery is recharged in 1h30, the remaining 30% in 2h. It can be relevant for a “fast” charge to stop the charge at 70% when the charge current starts to decrease. Unfortunately, this type of charger does not indicate this mode change: the blue LED will only be turned off at the end of the cycle. However, you can implement a counter in the ESP32 that notifies the user after 1h30 that the battery is charged at around 70%.

How to know the battery discharge from the code

With a Li-Ion/Li-Polymer battery, we can estimate its remaining capacity based on its voltage. The board integrates a voltage divider bridge, connected to the pin GPIO35 , which divides by two the voltage \(V_{DC}\)

We can therefore know either the voltage of the battery \(V_{BAT}\) , of an external power supply \(V_{IN}\) or also of the USB \(V_{BUS}\) with the following formula :

The ratio is \(1.435\) . The following Arduino code is used to measure the battery voltage :

void setup() {
  pinMode(35, INPUT); // You should set the input pin
  Serial.begin(115200);
}

void loop() {
  float v_mes = analogRead(35);
  float v_dc = 1.435*(v_mes/4095)*3.3;
  Serial.println(v_dc);
  delay(100);
}

For your information, in theory, if we apply the classical formula, the ratio of the dividing bridge should be equal to \(1.37\) and not \(1.435\) :

But in practice, since the primary current of the divider bridge is very low given the values of the resistors (1 and 2.7MΩ), we can no longer neglect the leakage current to the pin GPIO35 . This is the main reason for the difference between the values. If we consider the resistors’ tolerance (1%), your ratio can be slightly different by a few hundredths.

Built-in protections

The uPesy ESP32 WROOM Low Power DevKit board has built-in fuses to protect your projects against short circuits :

• On the USB rail (\(V_{BUS}\) ), there is a 750mA self-resetting thermal fuse. It turns on and off automatically (a resistor that varies with the temperature, called polyfuse).

• On each of the rails \(V_{BAT}\) and \(V_{IN}\) there are 750mA non-resettable fuses from LittelFuse \(F_{BAT}\) and \(F_{VIN}\) . If they blow off, you must put in new ones because the board will no longer be powered. Fortunately, there are footprints on the back of the board for this purpose.

  

There is globally less protection than on the uPesy ESP32 Wroom DevKit board to limit the power consumption as much as possible and thus obtain a very low power consumption in Deep Sleep mode. There is no protection against overvoltage on the rails \(V_{BAT}\) and \(V_{IN}\) or reverse polarity protection on the battery.

v1.2 (February 2023)

• Modify the board to be assembled in the uPesy factory (schematic is the same)

• Use 0603 passive components instead of 0402

• Move the 2 push-buttons

v1.1

• Change the maximum current charge from 350mA to 500mA.

• Change the fuse’s rating from 500mA to 750mA.

• Add a 100nF capacitor for the internal measurement of the battery voltage by the ADC (GPIO35 ).

• Change resistors value on voltage divider for GPIO35

v1.0

First release